This is the so-called "Stand Your Ground" law
Florida Statute 776.013-(3):
A person who is not engaged in an unlawful activity and who is attacked in any other place where he or she has a right to be has no duty to retreat and has the right to stand his or her ground and meet force with force, including deadly force if he or she reasonably believes it is necessary to do so to prevent death or great bodily harm to himself or herself or another or to prevent the commission of a forcible felony.
So let's even(for the moment) pretend that Zimmerman's story that Martin threw the first blow is true. Not that he attacked him from nowhere, because that's demonstrably false. But we technically don't know that Zimmerman made the first physical contact.
What we'll be looking at, is criminal menacing laws. If Zimmerman was engaged in an unlawful activity, Stand Your Ground no longer applies.
A person is guilty of menacing in the second degree when:
He or she intentionally places or attempts to place another person in reasonable fear of physical injury, serious physical injury or death by displaying a deadly weapon, dangerous instrument or what appears to be a pistol, revolver, rifle, shotgun, machine gun or other firearm;
Jumping out of your truck and chasing after someone with a gun at your waist certainly places that person in "reasonable fear of physical injury, serious physical injury or death". And we flat out know Zimmerman ended up making good on the last one.
If Zimmerman is guilty of criminal menacing(and even most of his defenders seem to concede he did things which qualify) I do believe this law does not apply to him.
Note: This is the opinion of someone with no legal training. Anyone who actually does hold a legal degree, please refute/validate this. I have yet to see this subject even broached in the media.