Basically, one person just stands there while the other one climbs all over them, like a koala climbing around a tree. It helps a lot if the 'koala' person is petite, and the 'tree' person has strong legs
The 'koala' person starts out hanging off the front of you. Then without touching the ground, they have to climb the whole way around your torso . . . up over your shoulders . . . down your back and through your legs . . . and then end up hanging off the front of you again.