is a critical part of SA all the way back to the original patent. The action of the trail wrist must be pure extension/flexion. The point of IMA drills #s 1 and 2 are to train you to a pure extension/flexion movement. NG and others have their own ways of training you to do this.
FYI - Though drills 1 & 2 have you do this movment actively (in order to train the movment) this is NOT something you do actively in the full swing (reference Scott's comments for drill #3).