Teams have 6 players on the ice. One of those players can but doesn't have to be a goalkeeper. Thus, when the game is nearly over, the trailing team will often exchange their keeper for another skater in an attempt to score. This gives them 6 skaters to their opponents 5 and generally results in a lot of offensive pressure and sometimes leads to the game tying goal. Of course the downside is that if the opponents get control of the puck, they have an open net to shoot at, like in this video.