Two suggestions for dryland training for bumps here. The first is a classic with Jonny Moseley. But it's for zipperline competitive bump skiing, so Jonny isn't rotating his skis left and right much at all since that would slow him down. I suspect, since you mention pivot slips, that you are interested in rotating the skis without rotating the upper body. But from comments in this thread it seems that your RoM doesn't allow you to rotate feet/skis without rotating the hips. You could do some version of Jonny's extreme drill with that rotation, once you get the rotation going in the first place.
Jonny Moseley dryland training 2001
And to get to that point, you could first do ladder drills. Here's a fantastic set of 25 agility drills on a floor ladder. You could create drills for yourself that involve rotating the feet/legs but not the hips as you make your way along the ladder. Drills like this, on the floor in the gym, could certainly help you increase your foot/leg rotation speed. You wouldn't need to take a shovel to a hill and cut out steps like Jonny did. But that option is always a thought.
25 Agility Ladder Drills
One more. This one addresses the rotational upper-body/lower-body separation itself, which isn't addressed in the two videos above.
Dryland Counteracting.
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