Definitions by Bam its Dan!

Dirty Snowball 

When a male is performing intercourse on a female (missionary) and defecates into his on hand; and then while continuing to thrust at a steady pace he throws the fecal matter onto the female’s stomach with the intention for it to splatter/explode much like a wet snowball. The male may choose whether or not he would like to scream dirty snowball in celebration of his great accomplishment.

once the fecal matter smacks the female in the stomach, you have completed the "Dirty Snowball"

Dirty Snowball by Bam its Dan! December 10, 2010