Here's your answer:
The Pivot (in a swing) is unwinding hard from the top—but NOT straight to the finish!
The PIVOT....from the ground up...must stop and let the following pivot components pick up their speed.
So, #1 the left leg, straight or bent/ with or without the foot being flat on the ground, has to halt the pivot somewhat down near impact.
It had better POST UP or something!
Now, as for the FORCE from the left leg—coming from the ground (UPWARD), on the way to HALTING the pivot—ADDING to the clubhead speed by speeding up the left shopulder up/right shoulder down action.....
Yup.