Imagine a child on a swing. When you push them, you do it from directly behind them (90 degrees to their motion)
In hitting, now think of your hands (Pressure point #1) as the child on the swing, and the left arm as the chains. Same concept, push at 90 degrees to the motion, but on an angled plane, rather than a verticle one as in the swingset.
Extensor action keeps the chains straight.
In swinging, the picture is a bit more abstract because you are loading at 90 degrees to the plane of the left wrist cock, or more exactly really 'the' plane which is between the arms. The only way to do this is to 'allow' rotation of the left arm in the backswing (startup swivel). This generally gives the 'feel' of getting the left thumb, and even the left shoulder under the shaft such that the left thumb is what loads against the clubshaft, under it. A clear difference in feel due to the rotation of the #3 pressure point, which would then feel on the side of the shaft, vs. behind the shaft with hitting.
Edz,
Had to incubate some on this (I drew a swingset pic so I could "see" it). It seems like the pushing of the child on the swingset is "in-line" with the motion (zero or 180 degrees), but 90 degrees to the chain. That makes sense to me, as does the analogy of the the left arm representing the chains and PP#1 being the child.
So I interpret it as loading the entire lever at 90 degrees to the lever and directing that loading down the inclined plane.
Does that sound accurate?