A common question that is asked by a first-time hockey watcher is, "The puck is still in play, so why are they substituting?"

In the olden days, "changing on the fly" was a curiosity. Now it is a deadly offensive weapon. A player skates within six feet of one end of the bench (where a legal substitution can occur) and is replaced by a teammate coming off the other end. The team nets about 25-feet of ice in the exchange.

The Stanley Cup champion Edmonton Oilers choreograph their line changes to perfection. But line changes are important only on offense, but also on defense.

In the graphic below is Harvard's second goal last Friday night against Clarkson. Don Sweeney (2) dumped the puck into the Crimson's offensive right-wing corner. Sweeney, Josh Caplan (7) and Craig Taucher (10) peeled off and headed for the bench while Paul Howley (9) and Ed Presz (8) chased the puck.

Howley and Presz, left to take on the entire Clarkson team, were able to wreak enough havoc to win control of the puck.

Meanwhile, all five Clarkson players were at the boards expecting a wraparound pass.

Thus, when Taucher's replacement Tod Hartje (25), came on the ice, no Clarkson forward was within 30 feet of him. Hartje got the puck to Presz for an easy goal.