When I encounter big troughs between bumps I think about two primary tactics.
1) lift my tips right before impact so tips do not jam into face of bump
2) Avalement! or anticipate the impact and retract the legs actively a split second before impact which also helps with #1. Avalement, a French term from the 60's and 70's meaning to swallow or actively absorb by enlisting the stomach muscles to pull the legs up as apposed to relaxing the legs to absorb the bump.
Trying to skid through this type of bump run has never worked too well for me. Speed control seems to work best by using very tight quick turns at the crest of the bumps to check speed before extending down the back side into the trough pulling the feet back to re-center then actively pushing the feet forward and lifting the tips to safely clear the abrupt wall of the next trough and repeat.
Sometimes when the trenches are particularly abrupt or deep I will actually hop/extend aggressively before I retract aggressively just to clear the trough and make it to the top of the bump where I can check my speed. and repeat.
It is kinda like hopping down a stair case two footed and skipping every other step. You absorb the landing then aggressively extend your legs to the next landing then actively absorb your landing and repeat to the next step.