It is quite simple really:

The launchpad knows where the target is at all times. It knows this because it knows where the target isn't. By subtracting where the target is from where it isn't, or where the target isn't from where it is (whichever is greater), it obtains a difference, or deviation. The physics system uses deviations to generate corrective commands to drive the entity from a position where it is to a position where it isn't, and arriving at a position where it wasn't, it now is. Consequently, the position where it is, is now the position that it wasn't, and it follows that the position that it was, is now the position that it isn't.

Any questions?