Pay attention to the fact that you can get off a pot when you're sitting on it, so you don't have to be standing on it to get off of it.
The idea behind 'shit or get off the pot' is to not sit idly while your life goes by you. When you're on the pot, take a shit, or don't. With any situation in life, you can either do something, or not, but you shouldn't put yourself halfway into a situation (sitting on a toilet and dropping your pants) and then not following through with some sort of action (shitting, or leaving). Randall used this phrase in Kevin Smith's Clerks.