The ammount of pop you give depends on the quality of the pipe. In PA most of the time our pipe walls arent anywhere near 90 degrees straight up, more like 80 degrees or so. Therefore to avoid airing out of the pipe onto the deck you have to pop quite a bit. On better pipes though the walls are at 90 degrees the ammount of pop will only be minimal enough to keep you landing at the top of the wall.
Pumping is pushing down to increase your speed when in the tranny, the curved area between the top of the wall and the flat bottom of the pipe. I first learned to pump skate boarding, but on skis i first learned it on the little bumps the pizza/french fries skiiers make from carving back and forth and making big ass bumps. Basically on the bumps you just push down after you hit the lips of the bumps, thus keeping the bases on the snow and thus keeping up speed. In the pipe you do the same thing, just extend your legs and push down when you land, dont suck up the landing.
I suck at pipe but this is pretty much how it goes.
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If hate were people, I'd be China.