I always hear the overly stiff flex promotes low launch and push, fade/slice, and overly weak flex promotes high launch and pull, draw/hook. This has to depend on swing, no? I ask just because the only time I've hit a regular flex driver was a round I played with my brothers where I had to travel and could only fit my irons, not my 3W or driver. So I borrowed my brother's regular flex driver for the round.
My normal good shot with my stiff flex driver is straight or a baby fade, and my miss is a big push fade or slice. I had forgotten that the too loose flex shaft is supposed to promote a pull and draw/hook, and just thinking about the physics I thought the shaft would be bending too much as I accelerate which would open the face and cause a push-fade/slice. I set up accordingly, aiming off the fairway to the left, and took my regular swing, and hit sweeping fades all day that started right of my target line and faded pretty hard. Worked out all right actually, hitting a lot of fairways on the right side, but it was always a push-fade, not a pull-draw that I got.
Does that have to do with my swing? I've always struggled getting the face closed with the long clubs and recently have been working on really extending down and through the ball (instead of thinking of rolling to close the face, something I've tried before, which never works out well), and that works incredibly well, especially if I start the swing from the top smooth and easy and try to feel the power and explosion later with the extension down and through the ball. Maybe that means with a proper swing your hands are slowing down more before impact, getting the whip that closes the club face among other things, and in that case a too loose shaft will over whip there and cause pulls and hooks? But when I'm not getting the proper whip and extension to close the face the shaft whip/rebound happens later, so when I hit the ball that way with the too loose shaft I'm still in the phase of the shaft bending too much backwards, instead of whipping through the ball and closing too much, so I got push fades instead of pull hooks?