Originally Posted by dddd

I don't like the term "pull" as applied here, since it suggests pulling the frame's dropout ends apart with opposing force.


Frames actually need to be widened by working on one side at a time, so that each side's increase in width can be measured off of the other side while there is no force being applied to the reference side.


Simply wedging the two sides apart always results in one side yielding more than the other, since the frame tubes aren't identical in terms of dimensions, processing and heat exposure history during frame assembly. So you end up with a widened frame that positions the rear wheel off-center, with the critical frame alignment being lost in the process.
Bending steel tubing is simple for people who are familiar with bending steel tubing, who won't over-widen the frame or perhaps even buckle a frame tube. The force applied is focused on the thicker chainstay, with the seatstay more or less going along for the ride since it is springier due to it's smaller diameter. Arriving at the correct width increase on each side is generally a repeated process best approached from the state of too little rather than too much bending, though a final correction of 1-2mm of reverse bending tends to produce a more stress-relieved and robust alignment, and can be done using far less force than expected.