During anal sex, a cum bubble is produced when a male ejaculates into the rectum of his partner. Since it feels like flatulence, which is essentially a bubble of gas in the rectum, it was thus named.
To perform a cum bubble can be done a number of ways, best example is: when you and you female partner are knockin boots you both cum and as you pull out she quiff(pussy fart) causing her to make a cum bubble
After me and my girl busted a nut I pulled out she pussy farted and I show her make a cum bubble ........ Then it burst....... What a mess