Probably because it is the pressure point that actually monitors your three imperatives. The other ones all are ways to monitor power (PP#2 and 4 for pulling force, PP#1 for driving force). But you can be totally into 1,2,4 and still be having clubhead throwaway. PP#3 wlll never lie to you about throwaway. Only PP#3 tells you when you have succeeded in sustaining a flat left wrist, because it sustains the lag. And it's tracing the straight plane line.
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