Per 2-E "Notice this - the ball acquires only 70% of the Clubhead "approach" speed (so there must be speed) but 100% of the Clubhead "separation" speed (so there must be resistance to deceleration)...Prestress (Acceleration) stiffen the Clubshaft for consistent (minimum to maximum) resistance to Impact Deceleration."
Although Sergio's prestressed shaft looks (and is) impressive and probably helps increase ball speed, it doesn't happen tha way it's indicated in a few of the posts here. The prestress that we see on these pictures is more or less completely unloaded a long time before impact. And it's probably reloaded and unloaded a second time before impact. Shaft loading profile depends on swing technique. See this link. The first graph that appears has to loading bursts and is the most typical loading and unloading pattern:
Please also note that the shaft is completely unloaded at impact. Only centrifugal acceleration and mass velocity is at work, and the toe of the club is pointing down. Don't underestimate the power of centrifugal force regarding impact deceleration resistance!
The prestressing of course happens because active torq is applied. The torque causes a clubshaft bend that has a similar effect as an increased wrist cock. That in itself will contribute to increased swing speed.
Back to the pictures of Sergio: With the same swing and even stiffer shaft, the torque in the wrist cock would increase because the shaft wouldn't bend as much. That torque increase would in fact lead to reduced swing speed - as in clubhead throw away. As a general rule, big wrist cock (with a help of shaft bend) early in the swing will produce increased swing speed. Torque might increase swing speed if it is applied very late in the swing (as for hitters?)but that has no relation to what we are seing in this pictures.
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