An IWB (inside the waistband) holster of the kind Zimmerman had is nothing more than a nylon pouch attached to the belt with a clip and held against the body by pressure. They are generally worn behind the hip so they remain out of sight when wearing an open jacket. The short jacket he was wearing would have ridden up past the gun with his arms raised in defense, and would not have been a factor in the draw.
To draw a small, relatively snag-free gun like George Zimmerman’s Kel-Tec PF-9 from an IWB holster positioned behind the right hip while mounted, all the person on the bottom has to do is rotate their hips counterclockwise just three inches. This creates enough space to slip the right hand in the gap he’s created with the rotation, easily draw the weapon, and fire from contact distance.
It would take 1-2 seconds to do this once the rotation begins.