You seem to be referring to the “Stand your ground” law, which was speculated about as a possible defense for Zimmerman early on in the trial. It turns out that he did not invoke “Stand your ground” (which generally removes the duty to retreat under certain circumstances), he was able to mount a defense based on good old fashioned common-law. Under common-law self defense, while he did have a duty to retreat he was unable to exercise it with Martin sitting on his chest.
And just to pre-empt the inevitable “But why didn’t he just not get into the argument to begin with???”, remember - it wasn’t illegal for Zimmerman to get out of his truck, follow Martin through a public place, and start an argument with him. The clock would have started ticking on ‘duty to retreat’ immediately after the first punch was thrown, and according to Zimmerman’s side of the story he WAS supposedly trying to leave the scene when he ended up on his back with Martin on top of him.
