Volume hand question

How can I set the left hand calibrations to get a clear stop and start of notes while also having some room for a crescendo? It seems like with the Theremini, if I can only get a clear stop/start to notes if the range is very small - like a couple of inches from the loop and then maybe a few inches higher than that... if I try to set the far setting with more space, I can't get a clear start or stop - just kind of a 'wwwwaaaaah-wwwaahh.'
Hopefully I'm describing this right!
Any advice??
Thanks!
T4E

What about placing your hand below the antenna and cntrolling the volume with a sort of opening and closing motion almost like plucking strings. I've seen Pamela Stickney play a snappy walking bass line like this on youtube. 

First of all it is about practice. The volume hand is faster when it flips upwards (practicing without moving the arm).
Maybe the Theremini is sort of laggy which is no help here.
Btw. subscope theremins feature velocity..

.. and there is an add-on for the Etherwave.

I don't even know yet what 'Velocity' is... I'll look it up and hopefully I can do an add on for the Theremini?
I've tried the 'plucking' movement. It works a little bit but is only somewhat clean if I have a very short range for volume.