Upon seeing this game being played recently (on the top deck of a bus) I assumed that it was merely a charmingly violent local variation of the innocent childhood game of 'who can spot the most things whilst travelling'. After some research, however, I discover that it is seemingly played world-wide with more variations than you can shake a stick at.
The version I saw being played is as follows. Players sit in a car (or bus) and as they travel they keep their eyes open to the cars passing them in either direction on the road outside. The idea is for players to spot certain car types, shout out a determined phrase, and if they do so first then they get the chance to physically punish the other player according to the type of car seen.
So in my version, if you see a Mini car you shout, "Mini Dig" and if shouted out first you can dig your opponent(s) in the ribs. If you see a Volkswagen Beetle then you shout, "Beetle Bash" and then you can bash the other person in a place of your choosing. The highlight in the game though (at least in my local version) is that if you see an old fashioned Volkswagen Beetle you shout out, "Herbie" and can continue striking your opponent until the next qualifying vehicle is seen. Marvellous fun for all.
The variations on this game are legion. One of the most common is the addition of the colour yellow which adds points / punches. So if the mini you spot first is yellow then you can make two digs rather than a solitary blow. One of the most obscure is extra punches for bald-headed drivers. I particularly admire the following rule found on the Facebook group devoted to the game:"Also one of the rules should be that you can only call a yellow car whilst in another form of transport. Me and my friends added this rule after kicking the shit out of each other in a carpark"