This term is very similar to the term "How's it hangin' " The only real difference being that how's it bangin' is less formal than it's counterpart. The "It" in question is very contextual to the situation and could really mean anything.
A casual sexual relationship between friends.
Did you hear that Ash and Jade are hangin n bangin?
When you have sex with many different women within a period of about one month
Jasper: yoooo Luke I heard you were Bangin like a boomer recently!
Luke: yea I hope I didn’t get any std’s!
An act not limited to, but includes placing a wager on a race horse and winning that wager.
Tommy is bangin' horses really well today.