This is an excerpt from Hockey Coaching Bible, The by Joseph Bertagna.
No defenseman likes to get beaten one on one. The most embarrassing moment for a defenseman is when a forward skates by him. To compensate for this, defensemen tend to back up into the offensive zone and allow the opposing player time and space with the puck (figure 8.1). All players want time and space to make plays, especially when entering the offensive zone. It is very difficult to be a good defenseman if you don't have good gap control.
The defenseman (D) maintains the gap as the forward (F) moves up the ice.
Gap control usually starts when a defenseman leaves the offensive blue line. Often when the puck is in the offensive zone, defensemen will stand near the offensive blue line regardless of what is happening in the zone. The blue line is there for one reason and one reason only - offsides. The only time a defenseman should be standing on the offensive blue line is when his team has complete control of the puck. This will allow more space for the forwards when they have the puck as the defenseman becomes an outlet for them.
The blue line does not determine where gap control begins. A defenseman's gap when leaving the offensive zone should be established by how high the opposing forwards are. The longer the play is in the offensive zone, the deeper the opposing forwards tend to be. It is much easier to establish a good gap leaving the offensive blue line by using the opposing forwards as a measuring point. By establishing a good gap, the defensemen can slow down the opposing team's transition game. When the defense can slow down the opposition leaving their defensive zone, they allow the defensive team to apply back pressure (back pressure is when the defensive team's forwards are able to pursue the puck carrier from behind; see figure 8.2).
Forward on the defenseman's team provides back pressure on the opposing forward who has the puck. The defenseman's ability to slow the forward helps his teammate provide pressure from behind.
The gap established when leaving the offensive blue line is the gap established through the neutral zone and entering the defensive zone. A good gap leaving the offensive blue line allows the defenseman to come through the neutral zone and enter the defensive zone with speed. It is difficult to play a one-on-one attack if the defenseman has to slow down as he enters the defensive zone. While he's slowing down, the opposing forward is usually picking up speed. That is when a defenseman is most susceptible to getting beaten one on one. A good gap allows the defenseman to maintain the same speed as the opposing forward. It also will slow down the forward entering the offensive zone, making him less dangerous. It is difficult for a forward to create offense when a defenseman has established a good gap entering the defensive zone.
Also, if the forward is flying out of the zone, the defenseman might have to get going quickly by first skating forward and then pivoting backward once the proper gap is established. And finally, with regard to vertical gap, the defenseman wants to maintain the good gap from blue line to blue line. This requires that he judge the forward's speed and adjust his own while using good C-cuts, not crossovers.
Sometimes the forward might be skating very fast but in more of a serpentine route. In this situation, his velocity made good (or VMG, a common sailing term) up the ice will not be as fast as it looks. Defensemen must not be fooled. They must judge their opponent's VMG and adjust their speed accordingly.
When talking about gap control, people immediately think of the gap as the space between the forward and the defenseman vertically. What often gets overlooked is the lateral gap. When a defenseman has established a good vertical gap, most forwards will then take the puck to the outside. The mistake most defensemen make is to keep backing up as the forward goes wide. Once the defenseman does this, he loses the advantage he had by establishing a good gap. He's now given the puck-carrying forward time and space to make a play.
Therefore, it is just as important to keep a good lateral gap as it is a vertical gap. The wider the forward goes with the puck, the wider the defenseman should be (figure 8.3). The defenseman should always try to keep his outside shoulder in line with the forward's inside shoulder. This will prevent the forward from cutting back inside and will force him to the outside. There, the defenseman can pivot and take the proper angle at the forward.
Maintaining good lateral gap control means the defenseman moves wide with the forward.
Eventually the defenseman will run or angle the forward into the boards. Good gap control by defensemen will cause turnovers inside the defensive blue line and help the defensive team's transition game. Unexpected turnovers are a great way to create offense for a team.
Another way to establish good gap control is following the play up ice. Once the puck is broken out of the defensive zone, the defenseman's responsibilities are to either jump into the play to create an odd-man rush offensively or to follow the play in case a turnover occurs, in which case he needs to create a good gap defensively. The defenseman's job isn't finished once the puck has left the defensive zone. He must always put himself in the best position possible to play defense and establish a good gap. An unexpected turnover in the neutral zone can quickly turn into a scoring chance for the other team. If the puck is turned over in the neutral zone and the defenseman hasn't followed the play up ice, it will be very difficult for a defenseman to establish a good gap because of the amount of space between himself and the forward. The defenseman must always be conscious of creating a good gap. It is one of the most important aspects of playing good defense.