Whoops, I was unaware of that >_<
sry Inversion!

I believe instructors in high school use the phrase "pull the sound out" to discuss the idea of projection to students that age. Projection is all about technique (similar to what jthurman discussed above). I never really talked much about this idea until this year, I re-taught technique to the keyboards and the sound does carry much further. It creates the sensation of volume but it really just projects further.

I think they use the phrase because they don't want students to do the opposite of the action, which would be to keep the mallets on the bars after contact has been made. My instructor actually used the analogy of pretending the bars were hot and you couldn't keep the mallets on the bars.

I think percussionisto has the right idea. What the director is doing is merely giving a visual representation that the student can use to produce the result that the director wants. I don't think the director has time to explain the physics of percussion like: the concepts of force, aiming below the surface of the bar/head, how microseconds matter when it comes to how long the mallet/stick maintains contact with the head/bar, etc.

If a student is truly interested in learning all about these fascinating yet difficult concepts to understand (I barely do at times and I've studied this at college for 4 years!), then the director can explain it to them. However, I think to make a quick fix, "lifting the sound from the bars" is acceptable.