I think I can help clear this up a bit. @Horton isn't to far off here.
I've skied with Brooks a ton and look to him a lot for coaching. What's really being said by Brooks and Seth is we're letting our feet swing underneath us while our body stays still, but its slowing that process down. If our feet swing to fast, we lose the line tension and our outward direction and puts our ski, body and handle path to the inside. We will only get so much swing now and it will stop and Our path now is going straight at the buoy instead of continually building and carrying our width all the way out to apex and creating an arcing turn. Remember from centerline to buoy is one big turn, we just need to control it.
Now how we do that; the term I was taught and might be a different way to think about for a lot of people is "feel that power through your legs", we're controlling or slowing down that motion of when the ski swings underneath us at the edge change, by doing that it puts us in the best position where we can allow the boat to pull us up into the highest swing possible thus giving us the ability to continually build width and ride that line up as high as we can into the turn. I Hope this helps.