Happy Slap - Un like the others above some true some false, happy slapping came about with the making of video cameras, its been about since the HI-8s and now moved on to mobile phones.
A typical stereotype is a person aged 14-18 years, waring a cap, hoodie or a stripy jumper with also market bought jogging bottoms with the known 3 stripes going down towards the feet also waring trainers like Reebok or Nike sometimes but rarely a skateboarder make like DC.
You'll know when they'll attack you as it happens as soon as they or you sit down and the mobile will be off to the seat on your left/right side or even right behind you if it happens in the street.
One way to combat this is to carry your own camera/mobile camera and just as or after they hit you get a picture of them give it to the police or what not stating you want to press charges as they attacked you.
You could do as shown above, catch them by suprise and hit them in-stead but if so make sure you hit them hard enough so that you can run.
This could happen any where,
1. you see 3 or more people walking along one or more with their mobile phone out of pocket maybe faking that their texting someone or playing a game
2. another one the leader walking ahead maybe a little impaticante rushing forwards.
3. they sit around you with the camer mostly off to your side aimed towards you
4. you'll hear them edge the one to hit on and then without knowing they hit you and your un-able to do anything about it.