[BulldogMcC]

This is easier than you think, the free blocking zone exceptions are Blocking Below the Waist, Blocking in the Back and Clipping.

Only an Offensive lineman in the Zone at the snap can do these blocks as long as the block occurs in the zone and while the ball is still in the zone.
All the below the waist blocks, front or back must be against a defensive lineman who was also in the zone at the snap.
The BIB can be against any defensive player in the zone at the snap.

I just remember that if the initial contact is below the waist, it has to be lineman on lineman, with everything starting and still in the zone.

Backs and {ends that start outside the zone} cannot ever do anything but in the frame blocking.