When a couple holds hands passionately. Usually involves some weird movement of the hands. Most of the time the two don't realize they're having hand sex until one notices. for example:
"what are our hands doing?" "I dont know" "It looks like they're on acid!!"
"hey hand... lets smoke something then rub up against each other"
J:"So,what'd you and jhett do tonight"
B:"Oh nothing, just had some hand sex while we listened to Zeppelin. Why, what did you and Jenna do?
J:"....same thing, but.... well we'll just leave it at that"
B:"HA! sure, sure... w/e"