How does rub or marinade actually seep into meat?

This morning I began crockpotting my first (ever) pork butt in an attempt to make pulled pork. In preparation of this, two days ago I smeared the meat with a spice rub and put it in a large sealed bag in the fridge. I did this because the recipe I was following stated that giving the rub a few days to "sink in" to the meat was crucial. This got me wondering:

Does rub/marinade actually penetrate into meat, if so, how?

The animal is dead, so I would imagine anything along the lines of "osmosis" or "capillary action", etc. would no longer be functioning.

To me, it doesn't seem feasible that rub/marinade would actually penetrate into a (dead) piece of meat. If it does, I'd like to know how, and how deep the rub/marinade actually travels.