More importantly who instigated the struggle. The cut on the back of Zimmerman's head is the real tell on this. An attack from the rear as Zimmerman was walking back to his truck. Or maybe Zimmrman heard Martin approaching and turned to face him before being knocked backward to the ground. Clearly it was Zimmerman laying on his back because of the grass and moisture transfered to the back of his clothing.
Either way, it supports Zimmermans version, and assumedly most of the six witnesses.
Looks like the cops made the right call in the beginning.
He could have easily slipped in his gumshoees on the wet grass.