71 is your typical 69 position but with a twist, two fingers in the butthole of your partner.
I was on top of Ali, we were 69ing, my balls were teabagging her chin, and I slipped two fingers up her ass to complete the 71. Then she gave me a rusty trombone.
A 69 between 2 people who have cocks. When one of the parties is about to cum, they signal the other person to lick a finger and shove it in their ass.
The postion in which there are 2 people doing 69 (person 1 and person 2) with person 3 underneath doing butt stuff with person 1 and person 4 on top doing butt stuff with person 2. 69 + 2 people hence 71.
Guy 1: Brooooo, I saw my step sis and 3 friends doing 71 yesterday.