In today’s NHL, line changes are critical

Changing the right players at the right time can be key for gaining an advantage during an NHL game. Here are four examples of what's happening during line changes for the Bruins and the Blackhawks during the Stanley Cup Final:

1. THE RULE: In hockey, players can be substituted freely and often do so on the fly. Players exit the ice through the door furthest from the puck while players enter the game over the wall to avoid a jam in the change area. What’s happening: Chicago changes its attacking line, putting its best players on the ice. Dave Bolland out, Patrick Kane in.

2. WHEN TO CHANGE: Coaches prefer to change lines when their team has possession of the puck. What's happening: Bruins attack, Torey Krug and Adam McQuaid head the bench

3. MATCHUPS: When a team changes a line with their best players, coaches often counter with their best defenders. What's happening:When Krug and McQuaid get close, Zdeno Chara calls for Krug and Dennis Seidenberg calls for McQuaid.

4. PENALTIES: If one player jumps to the ice before his replacement and tries to play the puck, his team will be called for a too-many-men-on-the-ice penalty. What's happening: As Chara prepares to jump on the ice for Krug, the Blackhawks steal the puck and start the counterattack. Chara jumps before Krug can get within 5 feet of the bench and tries to makes a play on the puck, resulting in six men on the ice.