When I trained to be an umpire this was how this was described to me: The defender must prove control of the ball. That means that once he establishes procession of the ball he makes a play or move to release the ball (voluntary release). That is why you will see umpires hold for a moment on tag plays to confirm that the defender has the ball.
We asked a long series of questions on tags, force play (DP's) and collisions with players and walls etc. In the end - until the defender makes a 2nd move while in control of the ball - it is a drop. The most controversial piece of this for our group was the collisions - if a fielder makes a running grab going towards the fence and hits it after even two strides without making a move to release the ball - it is a drop play on.
Finally bought it when it was adjusted to the catcher holding the ball and getting thumped on a tag. He has to complete the play and demonstrate control of the ball after the tag for the play to be complete. Running into the wall, force plays and other collisions etc. is the same thing.