Yeah, I stiffen up JUST before a jump. I might be totally "cool, calm & collected" but I've gotten into this habit of clenching all my muscles (but, my hands still go forward????) just before my horse takes off. It only happens on verticals and spreads but it's still annoying.
My instructor says that I'm trying to "make" him jump (not jumping ahead, but trying to give him some IMPULSION!!!) by almost seeming to jump myself as though I were on foot!