Done it with Sonya and with Nikki and Sonya in a 3some. It's very nasty in a good way lol. It's kind of like when the sex is so hot you don't care if the house is burning down.
Snowballing is passing cum back and forth between two or more people's mouths. As it gets passed, the amount of saliva and cum gets larger and larger (or snowballs).
( http://www.urbandictionary.com/define.php?term=snowball )