Ray Tensing, who faces a charge of murder, testified that he was being dragged by the left arm when he reached up and shot Sam DuBose.
Prosecutors told the jury that Tensing wasn't being pulled by the car and didn't need to fire the single shot at the head of DuBose.
Much of the July 2015 incident was recorded on the now former officer's body camera.
The death of DuBose, who was black, happened in the same yearlong period that saw several controversial officer-involved shootings -- including those of Tamir Rice in Cleveland, Michael Brown in Ferguson, Missouri, and Walter Scott in North Charleston, South Carolina.
Tensing, who is white, was fired from his job, arrested and indicted on murder and voluntary manslaughter charges. He was released from custody after posting a tenth of his $1 million bond.
He testified Tuesday, telling the jury that he had reached into the car to try to grab the car keys from the ignition but DuBose started the car and began to drive away.
Tensing said his left arm was caught and he started to fall.
"I remember thinking, 'Oh, my God, he's going to run me over and he's going to kill me,'" an emotional Tensing said.
The officer, who was 25 at the time, said he reached up with his gun and fired at DuBose's head, which he could see above the bottom of the driver's window frame.
He stuck to his story when cross-examined by Hamilton County Prosecutor Joe Deters, who showed Tensing several snippets from the video. Deters contended the video proves Tensing's arm wasn't caught, that he was standing when the shot was fired, and the car didn't accelerate until after DuBose was hit.
Tensing, who is 6-foot-3, disagreed and said his chest-worn body camera showed the perspective of a man that was being dragged.
If he's convicted of murder, Tensing faces life in prison. He also faces a lesser charge of manslaughter.
The jury is scheduled to resume deliberations Thursday morning.
The traffic stop
After making the July 19 stop, Tensing asked DuBose for his license. DuBose couldn't find it and was acting "squirrelly," Tensing testified.
Tensing tried to open the Honda Accord's door, but DuBose used his left hand to hold it shut. After that, Tensing reached into the car-- and at that moment, his body camera shook out of focus.
"Shot fired! Shot fired!" someone yelled moments later.
DuBose's car rolled for about a block before crashing. He later died.
The video has been a vital piece of evidence for the prosecution but it's hard to see what happens in key parts of the footage.
There is a little more than a second between the moment Tensing reached in the car and when he pulled out his gun. It's hard to hear the gunshot. There's a bang, but the camera shakes so much that viewers are unable to clearly see the shooting. In court prosecutors showed one frame at a time while cross-examining Tensing.
It didn't take long for Tensing to lose his job and face an indictment for murder.
Deters at the time called the shooting "the most asinine act I've ever seen a police officer make."
The University of Cincinnati later commissioned an independent review that found
Tensing had led his department in the number of stops and citations, and that more than 80% of them involved minorities.
In court Tensing defended himself, saying he often didn't know the race of a person until after he approached the vehicle window.
Earlier this year, the University of Cincinnati agreed to pay nearly $4.85 million
to the DuBose family, provide free undergraduate education to his 13 children, invite the family to take part in meetings on police reform and issue a formal apology, a news release from the family said.