You know when you canter you squeeze your stomach muscles when you go up and relax when you go down to keep yourself in the seat? (Or something like that... Hard to put in words.
It's a bit
of the same principal. You use your stomach muscles to absorb the shock, just like sitting canter.
Completely relax your thighs and let your stomach muscles take the impact. It's super hard to explain, but I promise you'll feel it.