The best way to understand a two-line pass is to think of an off-side at the blue line.  In much he same fashion as an off-side at the blue line, when you think of a two-line pass, no player is allowed to precede the puck over the centre red line when a pass is coming from inside his defending zone. 

The easiest way to remember what factors are used in the enforcement of a two-line pass is to think of the following statement:

 “Puck for the passer, skates for the receiver.”

This statement means that a linesman looks for whether or not the puck was released from the player’s defending zone or if it was released in the neutral zone.  If the puck is released from inside the defending zone, regardless of where the passer’s skates are when releasing the puck, then a two-line pass will be signaled by the back linesman (linesman closest to the defending zone of the team passing the puck). When you see the back linesman raise his arm and point towards the center red line, making a large ‘L’ shape with both arms, it means that the puck was released from inside the defending zone of the player passing the puck.

The key to note here is at the moment the puck was released was the passer’s stick completely over the outer edge of the blue line. The outer edge of the blue line is the edge closest to the center red line. Remember this. If the puck was released while still in contact with the blue line then a two-line pass will be signaled.  Once again, if at the moment that the puck comes off the stick, any part of the puck is still in contact with the blue line, it is considered a two-line pass.

Most people simply look at the white part of the ice surface. They will argue that if the puck is on the blue line it is a good pass. This is not true. The puck must be completely inside the neutral zone when it comes off the stick.

The reason this is so confusing is that if a two-line pass is called when the puck happens to have been in contact with the blue line, the face off usually takes place just inside the blue line. This is done just to show coaches, players and fans that it was definitely a two-line pass and to avoid arguments about calls that are too close to tell. 

Once the back linesman signals a potential two-line pass, the call is then up to the front linesman who is closer to the red line.  The front linesman looks for any players that are on the opposite side of the red line from where the pass is coming.  If the skates of the player are over the red line prior to the puck crossing the line, and that player is the first person to touch the puck, then that player is off-side and the face-off will take place at the spot from where the puck was released.

As long as the player receiving the puck at the centre red line has one skate in contact with the red line (on the ice, not in the air) when the puck completely crosses over the centre red line, then this player will be considered on-side and he will now be allowed to receive the pass.  Once again, to make it perfectly clear, at the moment the puck completely crosses over the centre red line, at least one of the players skates must still be in contact with the red line, or on his side of the red line, in order for him to be considered on-side.

Remembering that there is nothing simple about the rules of hockey, consider a situation which, although rarely happens, is one which a linesman must be aware of and prepared for. A player may have both skates over the red line but stop the puck before it crosses over the red line.  This is legal, at least for the moment.  The thing the linesman now has to watch for to is what the player does with both his skates and the puck.  Remember that the puck is considered to be illegal until this player brings back one of his skates and makes physical contact with the red line.  As soon as the player touches the red line with one of his skates the puck is considered legal and it can now be pulled over the red line even if at that moment both of the players skates cross over the centre red line first.  If the player keeps both of his skates on the other side of the red line and pulls the puck over  before he contacts the red line with one of his skates, then this will be considered a two-line pass.  The same would occur if the player passes the puck over the red line before he contacts the red line with one of his skates.  In this situation, the Linesman will call a two-line pass as soon as the passed puck coming from the player with both skates over the red line, passes the puck over the red line.  As soon as this passed puck crosses over the red line, the linesmen will blow the play dead.  The puck does not have to reach a player of the same team because the puck is still illegal and should be blown down as soon as it crosses over the red line.