X's and O's of Basketball has an excellent breakdown of that backbreaking play where the Pistons were able to get a layup when the Celtics desperately needed a stop or foul.
They have diagrams, video, the whole 9 yards. Check it out. (hat tip to TrueHoop)
Here's a blurb.
As you can see, because O2 (Rip Hamilton) comes off his screen to the 3 point line. This is important because it forces Kevin Garnett to make a decision. Should he switch to cover Chauncey curling off the double stagger? Or switch to cover Rip on the 3-point line? And he has to make the decision in a split second. My philosophy would've been to protect the paint first, so obviously KG made the wrong decision if we use that philosophy. But you could make the case that leaving Rip (who was the leading scorer for the Pistons in the game) open for the 3-pointer would've been wrong.