© PoliceBodycam footage showed the moment an acorn fell onto Hernandez's car.
Deputy Jessy Hernandez throws himself to the ground, rolls and then pulls out his pistol to fire a hail of bullets into the back of his car in bodycam footage.
He believed Marquis Jackson, who had been searched twice for weapons and was handcuffed, had shot at him.
Hernandez, a two-tour veteran of Afghanistan who saw no combat there, shouts "shots fired" in the clip.
The officer yells to his colleague, Sergeant Beth Roberts, telling her he is hit and was "shot through the car" as he scrambles for cover by a parked car.
Sgt Roberts then fired at the car, forcing Mr Jackson to play dead to avoid being killed.
In the footage, Hernandez tells Sgt Roberts "I'm good" as a person pleads with the officers but is told to "get back".
"It might have hit my vest," he says in the footage, then he retreats to another police unit that arrives at the scene with assault rifles.
Mr Jackson was not hurt in the shooting but says he has been traumatised. He had been detained on suspicion of stealing his girlfriend's car.
"All I could do was lean over and play dead to prevent getting shot in the head," he said on Facebook.
"I was scared to death and I knew all I could depend on was God! I ignored everything and prayed!
"Windows were shattering on me the whole time as bullets continued flying across me. I was blessed not to get hit by any bullets or get hurt physically but mentally, I'm not ok."
Hernandez told an investigator after the incident that he heard a suppressed weapon and he felt an "impact" on his right side.
But he was shown the clip of an acorn hitting his car from his own bodycam.
"What I heard sounded what I think would be louder than an acorn hitting the roof of the car, but there's obviously an acorn hitting the roof of the car," he said.
He resigned in December.
Eric Aden, Okaloosa county's sheriff, said his actions "were ultimately not warranted" but "we do believe he felt his life was in immediate peril".
Comment: There are obviously a myriad of factors that have converged to contribute to an incident like this - police hiring practices, high crime and violence, hysteria - however, taken together, it's just another example of how institutions in the US are collapsing due to being overseen by an incompetent and incredibly corrupt leadership.