Since there are no "Official" rules on the matter, here are my two cents...
I would say that with a normal hit you would nail the target in the chest (center mass) it would deflect the point and you'd keep on riding and normal damage would be dealt. On a Critical Hit I'd say you scewer your opponent in which case you'd have to drop the lance or be dragged off of your mount. On a Fumble I'd say that your lance shatters from the impact but doesn't actually do any damage.
I would say that with a normal hit you would nail the target in the chest (center mass) it would deflect the point and you'd keep on riding and normal damage would be dealt. On a Critical Hit I'd say you scewer your opponent in which case you'd have to drop the lance or be dragged off of your mount. On a Fumble I'd say that your lance shatters from the impact but doesn't actually do any damage.
