[EdZ]

The fundamentals of the 'machine' concept, as I understand them, are that force must be 'on plane'. Looking at each hand individually, they can be on plane, and be in any state of 'rotation' - that is to say, palm flat to the plane, palm perpendicular to the plane, or somewhere in between.

10-2B and 10-2D represent those two concepts.

In 10-2B, both palms are 'clapping' the plane - and this is clearest in a swinger's motion - rotation keeps them 'clapping the plane' as much as possible. The full roll of a swinger's horizontal hinge action, from startup turn to throughswing FULL roll - a steady, smooth, constant rotation. Letting physics 'win' over geometry. This is where most learning the game fail. They fail to trust rotation (and setup) and 'try' to hold the face square.

In 10-2D - the left hand is facing the plane as in the 'rotated' 10-2B (top of swing position), and the right is perpendicular - thus creating a 'T' - a battering ram - that rides the plane. See Alex Morrison's grip.

Any variation of either hand from 'flat to plane' or 'perpendicular' to plane leads to inefficient force - either off plane motion, or rotation that must be 'timed', or preset at address (hands to face). The hands either fighting each other, or in some state of turned/rolled that represents either a physics, or a geometry compensation, or both.

IMO - for swingers - 10-2B is the clear choice, however hitter's would do better to use 10-2D to start. Once hitting has been learned, moving back to 10-2B gives the added benefits of transfer power - forearm rotation - the benefit of a swinger's motion, with 'less' rotation.

Depending on the path of the hands, variations in the '90 degree' or 'clapping the plane' can be effective, however they are compensations and generally are only as accurate as rhythm allows.