Can be an initation rite or punishment:
at least ten guys need to crouch in a line letting their balls hang out, the new guy or dude who needs to be punished is dragged underneath the tunnel and will get a faceful of ballsacks. Voila, it's the teabag tunnel.
Moron: "I want to be in your crew, man! Leader: "Sure, but first you must complete the teabag tunnel!"