This is absolutely mind boggling
Fantastic exercise! My father, a biologist, used to talk to us kids about it. Proprioception is the same thing that tells you to move your foot when you drop something towards it, without having to look to know where your foot is. It's the reason you can catch a ball behind your back. It has a lot to do with coordination. (I mention, because this is a sports forum.) Your sense of your body in space. It has to be also related to "phantom limb syndrome," when people still feel a limb that has been amputated, does it not? And also how they're training people with prosthetic limbs to have feeling in them? The brain is a powerful thing.