He tapped his bumper.
I had a situation like that once, back in my younger years. Construction zone, lots of gravel on the road, and a sudden stop. I tapped the guy's bumper in front of me as I slid to a halt.
He immediately jumped out and called me, among other things, a-hole. Well, at that time in my life, I was determined that I wouldn't take that kind of remark from anybody so I jumped out of the car yelling "what the **** did you call me?".
I was ready to duke it out when, out of nowhere, a local cop walked up, looked at the two cars, and said "no damage, move along". I guess that took the wind out of both our sails! We both looked, agreed with the cop, and went along our merry way.
I don't think I could have remained calm like this guy did facing those kinds of actions, though I am a bit older now (and hopefully wiser, and definitely not as agile as I was 25 years ago). :)