It was a post by member khatfull, who described how he significantly straightened a warped rim by using the door frame and door edge of a heavy door (or metal gate). The frame and door edge is opened until it can slip the rim between, using the door as straight edge, close the door upon the rim, and a combination of pressure from the door and the person bending the rim, you work out the severe bends until the rim is a as straight as possible.

At least that's how I remember it.