Front leg at impact...

[FJRGerry]

My understanding of front leg positioning is it straightens as the hips rotate (but the knee doesn't lock) to act as a counter-force against the ball striking the bat. I tell our girls to imagine someone pushing at their hips from behind so they need to keep their front leg firm as the hips rotate.

 

[Mark H]

Whatever the bat head is doing right before impact is all you get. The ball is gone before the shock wave from impact reaches the hands so resisting impact with legs, arms etc is just a swing thought. May work just as any swing thought may work for any particular hitter at any particular time but it's not reality.