This is our old friend the scrum sled. One forward pack lines up in scrum formation and butts into it while the other pack stands on the back for weight. The sled is driven around for awhile, then the two packs switch. Repeat, repeat, repeat. Sometimes, if you've worked hard enough, you can see stars when you unbind from the last drive. Those allergic to physical exertion are advised to take up something more gentle.