Hey Arnold was a popular television show on Nickelodeon. In this show, Arnold had a secret hand shake with his friends where they moved their thumbs and went "wa woah!" The Hey Arnold is a sex act, where you insert your pointer and your middle finger in the vagina, then with your thumb move it on the clit like they did in hey arnold. The sound affect is neccesary to do the move right.
"You know that chick at that party last week?...Anyways, we were in my room and I busted out the Hey Arnold and she totally loved it"