Nothing, as Michelle Malkin explains:
So, what exactly do Stand Your Ground laws have to do with Zimmerman and Martin? Absolutely nothing, of course. Outside your own home, common principles of self-defense dictate that unless you have reasonable fear of deadly force or harm, you must flee if possible rather than use deadly force. But a “duty to retreat” rests on the ability to retreat. And “duty to retreat” was irrelevant in Zimmerman’s case because — pinned to the ground with Martin on top of him, bashing his head on the concrete — he was unable to retreat.