Start down is the change of direction from backswing toward the ball or aiming point.
How it is important is that it allows the player to store the energy of the flying wedges that were assembled in the backswing.
Why it is important is that it allows the player to use a trigger and delivery that will coordinate with their release type. Storing, delivering and releasing energy down, out and through the ball.
How hard? Not so sure what you mean.... unless the idea is a smooth transition...not jerky.
How fast? Swinger a bit faster than Hitter are the only measurements I can compare with, now....but willing to learn!
How much pressure? My thinking is it would depend on what release type you would use, sweep release feels like more pressure right from the beginning to follow through compared to a snap release. Drive Loading would feel like very heavy pressure.
I'm not anything and can't afford to stay in a Holiday Inn.
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"In my experience, if you stay with the essentials you WILL build a repeatable swing undoubtedly. If you can master the Imperatives you have a champion" (Vikram).
The reason you can't sustain the lag is because you are so eager to make the club move fast (a reaction to the intent of "hitting it far"). So on a full shot you throw it away too early, which doesn't happen for your short chip. (bts)