A command from one person to another to immediately humble him/herself and/or to desist from a detestable course of action. The phrase is an alternate, but more forceful way, of stating the slang, "Slow your roll". "Break Yourself" metaphorically alludes to a person as vehicle in transit that needs to BREAK suddenly in order to avoid an upcoming and potentially hazardous path or course of action.
Bitch, you is late with my money for the last time! You best break yourself 'less you fen to feel the power of my pimp hand!