In other words, the left leg must straighten at the correct time during the downswing if a golfer has an "optimum" kinetic sequence. However, the left leg doesn't have to straighten in a hyper-straightened manner (that causes a significantly positive o factor). Some superb golfers prefer to allow to allow the straightened left leg to still have a few degrees of flexion at impact/early followthrough, which means that the degree of positive O factor will be slightly less, and the left pelvis will be less upslanted at impact. I know of no reason why those two small variations in the degree of left pelvis upslant at impact should affect clubhead speed at impact.
I know of no causal correlation between driver distance and the degree of upslanting of the left pelvis at impact.
A more upslanted left pelvis at impact may allow a golfer to more easily hit upwards with a driver. However, according to Bobby Clampettt in his book "The Impact Zone", a golfer should always strike the ball while the club is descending to its low point - even when using a driver. Long drive competitors do not follow BC's advice because they seek different ball launch conditions when using drivers with very small loft angles.
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