Pop has two common uses:
First, and most common, is the amount of spring that board has when doing something like an ollie. A board with a lot of pop will be like a new rubber band. A board w/o much pop, will be like a very old rubber band.
Second, pop can reffer to the boost you get when going off a jump. A jump that is totally flat will have less "pop" than a transition, or a flat jump that has a bit of a lip at the top of it.
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