its just the amount of tricks you stomp in a run,
for example
in one run if you stomp just a 180, you get 1 grab
in one run if you stomp a 360 and a 540 you get two grabs
but if you have already unlocked a grab then do just a stomped 180 in a run, you dont unlock a new grab
if you wanna unlock them all fast and easy just edit the level and put all the jumps in a line