A modifier used to intensify a command. Slightly more powerful than "your ass." Appending "bitch" onto the end of the sentence further strengthens the emphasis as well as the condescending power.
From most polite to most offensive/aggressive:
"Please sit down if you would be so kind."
"Sit down, please."
"Sit your ass down!"
"Sit the fuck down!"
"Sit the fuck down, bitch!"