Since reading TGM, I would say that the drill teaches:
.3)How LAG is generated by the Pivot
.4)How LAG is effectively felt by the # 3 PP
1) The position of the hands and arms in catching LAG pressure against the # 3 PP
2) The quickness of the Pivot in sustaining the LAG
3) How to direct the LAG through the Plane
I'm sure there are other things.
ICT
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HP, grant me the serenity to accept what I cannot change, the courage to change what I can, and the wisdom to know the difference. Progress and not perfection is the goal every day!