Do they move much? Or do they just help dampen the high frequency bumps?
Personally I don’t really notice it moving at all, but it does an amazing job protecting your wrists / hands. It seems to move enough to absorb the jolt of the impact, but not the whole movement, so the front end still moves up and down with the terrain, but without battering the rider.
IMO they work way better than you’d expect for 20mm (IIRC) movement.
I rave about them and have generated loads of sales for them, everyone I know now has one on their gravel bikes!
I rode my brother Girvin flex stem in the (late) 80’s and it was not great.
Things have moved on a bit in the last 35 years or so. My brother had one as well….