I received a question about frequent pop ups to my site. How would you answer...here is mine
Popping up is staying on the back side too much. Make sure he strides to a point that he tenses his lead inner thigh and keeps his hands back. The weight should not stay on the back foot and spin against live pitching or in drills.
The HBH work should be about balance and learning to turn into the ball hard. We learn this by taking away most of the shift until they can learn to rotate their hips completely and create rotary power
Alternate HBH work with the walk up drill. Set a tee in front a couple of steps. Cross step in front with the back foot moving forward( step 1) then stride( step 2) and hit the ball. Make sure your get weight into the inside of the lead thigh but not beyond it. You can even drag the rear toe somewhat at impact as long as the hips are turning. The rear foot can be weightless at impact during this drill.
While this seems counter -intuitive to the goal of the HBH it is important to develop feel and athletism. Lunging is bad but so is staying back to much and having the early launch pull you back.
The bat and loaded shoulder unit have a certain amount of inertia. Stride momentum or weight shift just prior to launch overcomes the inertia ( which can exert a force rearward making it appear that you are dropping the shoulder).
Work on core strength because MLB players can over come the bat and shoulder inertia with strong abs and torso muscles and reduce their forward shift widening their timing window. \
A Jim Edmonds and a Garciaparra simply turn with hardly any shift while an Arod has more shift. You will need to work to find whats right for you but understanding it is key.