I could easily be wrong about the dialog, but going by that video's time:
0:39 cop: "What's the bottle on the floor there?"
0:41 DeBose: "Oh, that's a bottle of air freshener."
0:42 cop: "Bottle of what?"
0:43 Debose passes out bottle: "You can smell it. It's air freshener. It's not a liquor or anything."
1:47 cop: "Well until I can figure out if you have a lisence or not, go ahead and take your seatbelt off for me."
1:50 cop opens door, Debose reaches for ignition: "I didn't even do nothing, what are you--"
1:51 DeBose closes door, cop: "Go ahead and take your seatbelt off."
1:52 DeBose starts car then reaches for shifter with right hand
1:53 cop reaches into car and grabs at wheel/keys: "Stop. Stop!", DeBose reaches for cop's arm with left hand
1:54 cop grabs the seatbelt and pulls gun, DeBose sees gun and cringes away, both hands in the air
1:55 cop shoots DeBose
Now this is difficult to tell, but it seems to me the officer shoots before the car moves, but the car moving is what knocks the officer down. Look at the distance to the cop car behind them to tell. It looks the same distance at 1:54 before shot, 1:55 during shot, but further away at 1:56 when the cop falls. That's what I see.