Holding @ the base like a clamp, moving up and compressing (like pressing down).
I used to w/ my left hand, make a jelq like clamp, tweak my wrist up moving upwards, then back down, all while doing a base clamp w/ my right hand.
I seen someone in another video pressing @ the head doing the compression. Seems there are different ways it's being done.
DLD sums it up though, very quickly I may add