In corners, large packs of riders tend to slow down as they approach.

When tailgunning, you sit at the back. Before the corner, you let a small gap open between you and the field. As you enter the corner, you don't slow down like everyone else. As the pack exits the corner, they have to sprint back up to the speed they were at as they approached. You don't have to sprint because you've maintained the same speed you had when you entered the corner. You've effectively caught up to them, or rather, they're now back up to your speed and everybody's in the same position as before. Except, they've expended more energy than you because they had to sprint.

It's not as easy as it sounds. And CDR may want to correct my explanation.