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.

I don't think "pulling the sound out of the instrument" has anything to do with the sound in itself, however by doing it the motion before hitting the instrument changes when you think "pull the sound out". You tend to be much more relaxed when the motion is described as "gently pulling the sound out of the instrument" versus "hitting the note". I doubt if anyone actually believes you can literally pull sound out of the instrument but that mental approach translates to making a much better quality of sound from the instrument.

I think that's why people teach it. It's a mental trick. They got us.

This topic has been the subject of many, many discussions over the years and I would always take the suggestion of a person much more talented and experienced than myself. In this case, Mr. L.H. Stevens. Read his book Methods of Movement and you can apply that technique to all concert percussion instruments.

In a nutshell, with apologies to Mr. Stevens:
Close your eyes and pretend to be a marimba bar/vibe bar/timpani head/etc. Have a friend hit you three times with a mallet, being carefull not to change the velocity of the stroke. The first time, have them do a round-off back handspring and then hit you. The second time have them stike you and THEN do a back flip. The third time have them just hold the mallet 8-9 in from you, hit you and then return the mallet to its starting position.

Could you feel a difference? (If the velocity was really the same each time, then the answer should be "no") If you can't tell the difference, do you think a marimba bar could? "No". So what happens after a stroke has more to do with preparing yourself for the next stroke than anything else.

The "lifting" idea is born from students who downstroke EVERYTHING with a ferociousness than leads to high velocity and a very poor sound. The opposite of this downstroke is a up or "lifted" stroke with less velocity. Telling a kid to "pull the sound out" a quick way to fix a barbaric downstroker, but you are correct in saying that what happens after something is already struck is meaningless.