Suppose you are making a gate and you need two horizontals to be exactly the same length, but one turns out a little longer than the other. You have already forged something -- a tenon for example -- on the ends of the bars. You can't cut the bars. How can you shorten one or lengthen the other?
The answer is simple: draw out or upset. The shorter bar can be lengthened by hammering uniformly along its length, cold or hot. The more you hammer it, the longer it will grow.
Or, the longer bar can be shortened. This is a little more difficult, but it can certainly be done. Heat the bar along a section somewhere near its middle to yellow-orange, then upset it. If there is a tenon on both ends, this upsetting can be done by placing the tenon on one end of the bar in a matching hole drilled in a 2" x 3" x 3" .5" square steel block on the floor, placing another similar block on the top tenon, and hammering downward along the axis of the bar. Turn the bar frequently, straighten it when it bends, and only hammer while the bar is hot enough: between bright orange and yellow. Overshoot a little, making the bar just a little bit too short. When the bar is a bit shorter than the target length, re-heat the upset portions and hammer them to blend them smoothly into the rest of the bar, checking the length as you go.
The only challenge in these techniques is to smoothly blend the drawn-out or upset regions into the rest of the bar so they look right.