It sits between the spring and the cylindrical roller piece in the hoof. Basically it helps spread the force along the spring rather than having it all on one point where the roller is.
Problem is, they fall out as they don't appear to be fixed in any way, they just slot in, and once they fall out they keep on falling out each time you try to put them back in.
I suppose this was a mod to replace them... ProJump, is this something you have designed and made yourself, or is it a FlyingLocust part?