According to head coach Joe Mazzulla, three controllable issues can force the Celtics into transition defense (a place no player, coach, or basketball-trained golden retriever wants to be).
Turnovers are obvious. A sloppy pass or a lazy, exposed dribble will lead to a dunk more often than not. Even the worst NBA players are expert finishers when given a numbers advantage and a head start.
A missed layup is the scourge of transition defense everywhere. When a guy goes to the rack at full speed and misses, his momentum carries him out of the play and into the photographers (dangerously!