It's my interpretation that 776.041 (2) (b) presumes provocation equal 'physical contact'.
(b) In good faith, the person withdraws from physical contact with the assailant and indicates clearly to the assailant that he or she desires to withdraw and terminate the use of force, but the assailant continues or resumes the use of force.
Provocation (sufficient to justify the other person resorting to the use of force) need not include physical contact. If two people are in a verbal argument, and anger is present, and one person moves in a way that a reasonable person would take as preparation to use force, counterforce is justified. Another scenario is a person throwing things at you, and yet another is a reasonable belief that a person is reaching for a gun or knife.
Nearly all people have a pretty good sense of what being "provoked enough to be justified in using force" consists of. It's those situations where you can use violence, and won't get in trouble for it.
Serino suggested to Zimmerman that Zimmerman's pocket reach might have set Martin off on his attack. I don't recall if Serino offered an opinion about whether the attack was justified. IIRC, Serino was looking for some reason beyond "just for kicks" that would explain Martin throwing the first punch. I don't feel any urge to probe that, as Martin spend a substantial length of time restraining Zimmerman in a physical predicament. That is, the issue goes beyond "first punch" and into a minute of physical superiority. Whatever the reason (for physical attack) was, it was reason enough for both the initial punch and the minute of restraint. Doesn't seem like the reason would be related to whatever Zimmerman was reaching in his pocket for.
That section of statute you cite allows an initial aggressor to "back off" the threat or use of force (which, in the universe of scenarios might be a non-contact action, for example a "Mexican standoff") and regain the ability to have legal justification for (later) use of force in self defense.