Originally Posted by
vasracer
Steel has memory meaning if you crank your axle nuts down squeezing the drop outs toward the hub, the drop out will want to return to their original position, causing an immense amount of stress and tension on the rear triangle. This along with bumps from the streets and the torque from pedaling can eventually crack your frame. Spreading the frame out will put tension on the lock nuts of the hub, eliminating any possibility of cracking the frame. (speaking from experience)
Im not going to be rude, but this isn't how metals work.
both widening and narrowing the rear opening will simultaneously stretch and compress different parts of the frame. so damage can be done in either case.
The real concern here is exceeding the metal's elastic limit, at which point it becomes permanently distorted (bent) and wont return to its original shape.