Another question you might ask is this: How can you release #4 and hold #1?
I think #1 will start to release as soon as you release #4, but there is motion and then there is power. If we talk about power and if you do the 4 barrel sequence you will start to apply power #1 after you've applied your #4. If you check this in slomo you should be able to find a geometry where you can do 1# then #2 then #3 in sequence after #4.
PS: I suspect that there's some increase in bend/cock in the right wrist in the downswing for those who do a snap release. The two hands aren't quite as synchronized as the book indicates and I think any player should make up his mind which hands geometry to use as a basis. A hitter should probably keep the right wrist frozen and sacrifice some lateness of accumulator#2 while a swinger is probably better off allowing the right wrist to cock & bend some extra before the #2 release. IMO.
Let me see... for a Swing the right heel turns the hip which turns the shoulders and Lags the club back and up! As the # 4 accumulator is loading the left heel comes down loading all 4 accumulators even more and Lagging the right hip, Lagging the right shoulder, Lagging the wedges and activating the Flail to an "arrow through the head position." There seems to be a lot of overlap to me guys and even more with my senior shafted driver! When possible, I will change that to a stiff shaft.
In Mr. Rieger's video, his discussion of "the Forward Press" is very interesting.
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!