I found the Sanford Florida Police Department's online material for Neighborhood Watch. And this is on page 3:
You will add your “eyes and ears” to those of the Police Department which cannot be everywhere, all the time, by keeping a watchful eye and open ear to what is happening in your neighborhood. You will extend their ability to provide security by reporting anything unusual or suspicious, 24 hours a day, seven days a week, so they can follow up on your leads. What you will
not do is get physically involved with any activity you report or apprehension of any suspicious persons. This is the job of the law enforcement agency.
Bolding is in original.
NW Program Handbook from Sanford FL PD (PDF)
George Zimmerman was in explicit violation of the rules of the Neighborhood Watch program the minute he got out of his car. And he was in implicit violation the minute he stuck a gun in his waistband, because he should never have done anything that put himself at risk.
I think it is worth noting that George Zimmerman was not acting as a Neighborhood Watchman. He was acting as an armed vigilante.