When you bounce on boxes, your kill/bounce goes up just like when you do it with enemies. You only get rewards when you kill enemies though (like when you kill 5 enemies in a row you get a life). So you bounce up until 200 (be it by bouncing on a box or bouncing on enemies), which is when the game starts giving you crystals. To keep the counter from resetting when you hit the floor, you spin and either jump or crouch when the spin ends. So then you just need to kill enough enemeis to give you the right amount of crystals. Why the game gives you crystals i don't really know, but that's how you execute it.