The left forearm rotation. It isn't a "must" if you are in the proper alignments/plane, but if you first make it "active", you can make up for a lot of mistakes and get the feel for what on plane rotation is.

Grip is a big factor. With 10-2-A it basically must be active, with B, it may, or may not be and with D, it better not be! Hinge action is of course also a big part (I am assuming horizontal).