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Defending the Blue Line

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A look at how the Bruins defend zone entries

NHL: Boston Bruins at Toronto Maple Leafs John E. Sokolowski-USA TODAY Sports

Data is all at 5v5 and via Corsica (up-to-date) or self-tracked (thru Dec 29th). Defensemen will also be referred to as backs.

Introduction

Quickly Google search “zone entries” and you will find that the hockey world is obsessed with them. Eric Tulsky, now the Vice President of Hockey Management and Strategy for the Carolina Hurricanes, deserves a lot of the credit for just how obsessed coaches, players, and fans have become over this small element of the game. Tulsky pioneered much of hockey’s research on “micro-statistics” during his time at SB Nation. You can find some of his best work here, in a paper for the MIT Sloan Sports Analytics Conference in 2013.

The importance of zone entries is simple; Controlled entries produce about two times the number of shots and goals per entry. Much of what happens defensively, occurs at the defensive blue line.

Fluto Shinzawa of The Athletic wrote about how the Bruins go about this in September. I highly suggest reading that article first. In there, Shinzawa compares the Bruins squash-and-slide tactic to more traditional entry defense, while also including some comments from Bruce Cassidy.

How Does the Squash-and-Slide Differ from Tradition?

In traditional zone entry defense, teams generally preach “receiving the entry.” That’s jargon for staying between the faceoff dots and forcing the play to the outside. This makes it harder for the opposing puck carrier to get into a good scoring area.

In the diagram above, the backs will stay between the faceoff dots, marking the puck carrier and the slot. The first forward back will also defend the slot, allowing the back to play the puck carrier once he comes close enough. The second forward back will support the strong side, “squashing” the play along the boards if the puck carrier chooses to retreat there. The weakness of traditional entry defense is that it gives the puck carrier time and space to make plays.

Even if the player can’t get the shot he is looking for, he can still take a shot with purpose and hope for a rebound.

A slightly more aggressive alternative is to have the F1 pressure the puck carrier, while the backs stay between the faceoff dots.

This forces the backs to communicate and be aware of the cross-ice pass, and step up if that occurs. F2 will support the strong side while F3 retreats to the slot.

The squash and slide combines those two ways of thinking. Rather than the first backchecker trying to force the puck carrier to the outside, who is skating forward and coming from behind, the back will force the puck carrier to the outside while the first forward back will protect the slot.

The backs act like a foosball table, hence the slide. They stay at the same depth and slide to the strong side together. If the puck carrier tries to take the puck wide, the back will look to “squash” the play along the boards. If the puck carrier tries to go inside, the backs and F1 will look to squash the play by pinching in. By applying calculated aggressiveness, they take away the time and space from the puck carrier, forcing him to make a play before he wants to.

Adding Context

Now that you have an understanding of the system, you are probably wondering if this is effective, right? Let’s first look at the big picture and move in. Right now, the Bruins rank 4th in adjusted shot attempts against per hour, 4th in expected goals against per hour, and 2nd in goals against per hour.

In other words, the Bruins defense is very good as a whole.

While they do give up more shots than league average from the slot, their shot suppression, presence in front of the net, and good goaltending has led them to phenomenal defensive results this season.

I’ve used passing data in a few posts so far this season, most recently when addressing the Bruins’ scoring woes. This data includes the last three consecutive completed passes prior to a shot, and where those passes generally occurred. If one of the last three passes was in the neutral or defensive zone, it assisted in the entry leading to the shot.

Ryan Stimson and Matt Cane explored using this data for evaluating entry defense, and found that this was a repeatable skill at both the team and player level. Furthermore, they are generally a better predictor of goals against than other metrics like shots, expected goals, and goals.

The Bruins are very successful at suppressing entry assists, only allowing about 10 per hour. Teams generally range between 12.5 and 17.5, leading me to believe that the Bruins are the best in the league in this metric. The Bruins also claimed to be first in an in-house metric to evaluate entry suppression last season, which helps the case. Unfortunately, because this data has to be tracked manually, we will probably never know.

While it’s safe to say the Bruins don’t give up many shot attempts from transition, what are the quality of the shot attempts the Bruins are giving up? Luckily, Stimson also did research on how offensive zone passes after entry affect shooting percentage. Using this research, we can weigh shot attempts by the shooting percentages Stimson found for the number of offensive zone passes after the entry. The formula is:

xSh% = ((0.039 * 0 Passes) + (0.081 * 1 Passes) + (0.156 * 2 Passes) / Total Number of Shots)

Let me clarify that this is location independent, which is a critical component to an expected goals model. Furthermore, the shooting percentages Stimson found were for shots on goal, where I am referencing all shot attempts. This is just a more attractive way of weighing these events.

The Bruins, unsurprisingly, are terrific here too, with an xSh% of 5.64%. The average from the 2015-16 season was 6.09%. It seems at this point that we can conclude that the Bruins are good at suppressing shots from entries while also taking away time, space, and passing options on the entries that they allow.

Adding More Content

Now that we have an understanding of the Bruins squash-and-slide, both tactically and the statistical impact, let’s look at some examples. Below is an entry from when the Blues visited TD Garden on the 17th.

This example is far from perfect. It seems as though the Blues pre-scouted the Bruins and adjusted accordingly, however, the Bruins did “squash” the rush. John Moore comes out to the boards, forcing the puck carrier to get rid of the puck. While Brayden Schenn retains possession of the puck, the Bruins can now set up in their defensive zone structure.

Without the squash-and-slide, the Blues would be in good position to middle drive. Schenn would drive straight at the back and towards the net. This would create space for the puck carrier to shoot, pass to Schenn, pass to Kyrou (33), or try to pass off of the pads.

While the Capitals put their own twist on things, this is an example of how they use a middle driver to create space for Ovechkin.

You can point out a few flaws by the Sharks there, but look at how the Bruins handle Ovechkin differently.

In the example above, Carlo is caught up the ice, so Krejci is handling his duties while Carlo fights his way back into the play. Instead of staying inside of the faceoff dots, Krejci challenges Ovechkin. There is no space on the outside for Ovechkin, so he is forced to move inside. This goes against all conventional wisdom, but the Bruins essentially forced Ovechkin into Chara. While Kuznetsov and Wilson seem to be open, the Bruins force Ovechkin to make a decision, where he eventually chooses to shoot. Although he got a shot off, it is less dangerous than it could have been.

While the sqaush-and-slide is very effective, it has it’s weaknesses. For one, it requires a lot out of its forwards. A lack of awareness and effort is amplified because of the aggressiveness of the backs.

In this instance, Chris Wagner tries to be a hero and force a turnover. Tage Thompson easily side-steps him, forcing the backs into a tough position. Chara and Carlo take away the strong side, opening up space on the weakside. Sobotka fills the space Wagner should be taking away. The Sabres now have a 2-on-1 with Carlo, creating a dangerous scoring chance.

In the clip above, McAvoy and Grzelcyk do a decent job of taking away the play to Josi, and forcing the puck carrier to continue to handle the puck. While Krejci was at the end of a shift and had a hard time getting back up the ice to assist Grzelyck, Marchand doesn’t take a stride after the red line. With a little more effort, he could have helped the defensemen “squash” the play in the high slot. Instead, the Predators create a dangerous chance and draw a penalty.

The squash-and-slide, among with practically any other entry defense strategy, struggles against underneath entries. This is similar to the drop pass on powerplay entries.

The closest forward comes underneath the puck carrier and receives a pass. The former puck carrier then drives the net. This simulates a middle drive against an aggressive defense.

Underneath entries require more speed and skill, but when executed properly, are very dangerous.

Conclusion

There has been a lot of information packed into this article. Here’s a quick summary:

  • The Bruins use a rather unique tactic to “squash” transition plays at the blue line rather than letting the puck carrier have time and space after entering the zone.
  • The Bruins don’t allow many shot attempts directly from transition plays.
  • The Bruins do a good job at taking away passing options on the rush.
  • There is no perfect strategy. Defense is about damage control and getting the puck back. Sometimes teams will create dangerous shot attempts, but the Bruins give up less of these dangerous attempts than most teams.

Entry defense is a small, yet important element in the game. Furthermore, the tactics addressed in this article were set around 3-on-2 and 3-on-2 with a close backchecker. There are a whole matrix of possibilities from 2-on-2 to 4-on-2. The philosophies set forth by the coaching staff along with the execution of the players is what leads to the Bruins success in this area of the game.