...it can't have been *that* long since this was last discussed...

Anyway, to nutshell it: you're trained in riding with stirrups. Back then, cavalrymen learned from ground up to do without in full combat. Easy as that, really; a lot of "impossible" acts are just a question of putting in enough training hours.

As for the impact, eh. Just loosen your hold on the spear-shaft if it seems to become a problem, and let it "glide" until it's safe to grab it again and extract. This is really what the stirrup, the couching technique and eventually the full-plate lance-rest did, in fact; hike up the "absolute threshold" of impact the rider-horse team could deal with before having to "let loose" for safety reasons.