Clenching involves you and at least 2 of your closest friends whom you have absolutely no sexual feelings towards. All three participants leave their clothes ON and squish into a twin size bed in a spooning position. Then proceed to bounce up and down/hump to the tune of "shots" by Lmfao while chanting "clench clench clench". Continue til exhaustion, then fall asleep maintaining position. Usually done when completely wasted. Often accompanied by feelings of "WTF" the next day upon reaching sobriety.
guy: did you get any last night?
guy 2: yeah, dude, I clenched these two hot roommates last night. They had ass for dayzzz.