He was knocked down after he shot him, and he was knocked down because the car sped off after he fired the shot (y'know, because DuBose's dead body slumped forward, causing the car to accelerate). The guy didn't try to run him over - the car sped off after he was dead.
Before.... After...
Seriously, who cares? Even if Tensing wasn’t knocked down, the assault occurred when DuBose put the car in gear.