I read a guide that Jed linked in the Balance on Jumps thread, as I wasn't really sure what was meant with the pop, so that guide came in handy.
In that thing they said that you should pop from your core and not push off with your legs, but even if you do that, wouldn't you still have a bit of a jump from your legs when you leave the ramp or w/e you're going off at the moment?
To clarify, the guide is saying to keep your core tight, but as already mentioned, pop starts from your legs.
Basically pop comes from your legs, but your core helps keep everything stable and in line as you pop. So your core isn't directly creating the pop, but rather helping to keep you stable while you pop.