James Hallett, 44, was off duty when his car collided with another man’s vehicle at a roundabout around 12.30pm on Friday.
The collision caused the other driver’s car to flip onto the roundabout upside down, trapping him in the car.
Mr Hallett said: “I was going around the roundabout when I got struck from the near side. He came off the A41 and lost control.
“He had been catapulted into the back of the car. His legs were still in the seat belt and his whole body was on his neck.
“He was upside down with his head against the roof.
“I broke both the rear windows , I got a knife and crowbar and attacked the seat belt before helping clear his airwaves.
“He was totally unconscious and blue in the face. He was struggling for breath. If you didn’t do something immediately he would have died.”
Mr Hallett, who works for Northolt fire brigade, waited for the emergency services to arrive before the man was taken to Stoke Mandeville Hospital.
The fireman added he was trained in dealing with such incidents and the scenario did not faze him.
However, he first had to deal with helping two passengers from his own car before putting out the smoke surrounding the other driver’s car.
“I helped the two people in the car with me out because they were in shock.”
Mr Hallett and his fellow passengers were treated for minor injuries.
Two Bucks fire engines were first on the scene.