As I understand it (having played it once), you pair up. One duo is shooting threes, the other duo is rebounding.
The shooting team alternate. Player one shoots, then player two, etc.
If you make your three, you get a point. If you fail to hit the ring, the other team gets a point.
The team rebounding cannot step outside the three point line (memory of rules is hazy here) nor block the shot, only try to distract. If they touch the opponent while he's shooting, the shooter gets the point.
Now, where the tips come in is a bit like volleyball. Each rebounder gets one touch of the ball if it misses, but doesn't airball. They can either alley it in, or set it up for their team-mate. You must be off the ground when you touch the ball. I think that generally you're only allowed to touch the ball with one hand, but you can obviously vary this depending on skill. If the rebounders can tip it in, they get a point.
Every multiple of five collective points, you swap roles - the rebounders become the shooters, and vice versa. If the score is 3-7, you switch. If it's 12-8, you switch. Easy. First to 11 or 21 leading by 2, whichever you prefer.
As shooter, obviously your goal is to make your shots, or if you must miss, make sure it's going to be tough for the rebounders to tip it in. As a rebounder, your goal is to distract the shooter and then get into a position where you can set up your teammate or tip it in yourself.
If I've got any of this wrong, I'm sure someone can correct me. I'd never heard of it until this year, so I don't know how common it is.
You can, of course, customise it to allow bonus points for tip dunks or whatever else.