Start the downswing from the ground up?

what happens in transition that moment when the swing direction of the club appears to pause & the body's motion changes direction. is the same in 99.9% of PGA Tour Pro's swing motions, they will though have differing 'feels' of how they achieve a good sound transition.
{the commonality is in most efficient swing motions the transition move in the lower body starts before backswing reaches it's apex, dissociation of movement between the pelvic girdle & spine}

assuming a good turn to the top (not sway), still in posture angles with vertical height steady.

to get that dissociation between the rotation of the hips & the rotation of the upper body in transition the first motion needs to be some weight pressure into the lead leg, lead thigh/knee then re-rotates back to target this puts the joints of the lead leg back inline so the leg can provide resistance for the swing to turn around the lead hip socket.

in a face on view of the swing & from the outside of the lead leg imagine there's a vertical 'line' going up to hip height.

at address set-up there will/should be a small gap between the lead hip & that line.

in the turn to the top (not sway) the gap between the lead hip & the line will widen some. (if from this point at the top of the backswing the lead hip was just rotated/turned & cleared the swing motion would be is some difficulty)

as the hands approach shoulder high press some weight into the lead foot (as this happens it's important the pelvic angle is maintained) the lead thigh will move back in line as it does so it will take the lead hip upto & touching the line, (this small bump & sit motion will also bring the arms club trail shoulder downwards not outwards, OTT) then the lead hip rotates/turns & clears against the resistance of the lead leg, head stays back of ball as hands lead a forwards leaning shaft then clubhead through impact.

imagine the same above sequence looking at a dtl view of the swing.

at address imagine a the vertical line drawn from hip height to the ground against the butt.

as a sound backswing is made the trail front pants pocket turns towards that line (not sways away from target) & the back trail pants pocket turns behind the line towards target - at no point from address to the top of the backswing is there any gap between the butt & the line.

at transition (that weight pressure into the lead leg causing the little ways bump to target) from the dtl view the butt again stays on the line - as in the previous face on view this move closes the gap to between lead hip & the other line to close up.

then the rotation/turn of the lead hip clearing means from the dtl view we see the pants lead back pocket turn through the line the butt has been on - there has been no gap between the butt & line all the ways from address to swing motions finish.

if anyone interested enough (still awake) to see footage to have visuals to look at post below, if so I'll post them in the experts section.