I hope it's OK if I jump in here, "123Pugsy" is correct...I will try and explain how things like this happen...when you weld, you create heat, the heat expands the metal, when it cools it shrinks more than it expanded when it was hot. So, when you planish the metal (hammer and dolly) what you are doing is stretching the metal back to it's original thickness. Think of it this way...might sound stupid but it may get the point across...when you tenderize a steak by hitting the meat, the steak gets thinner, but it also gets wider...(sorry but it's the only similarity I could think of at the time). When the metal is back to it's original thickness the warping should be minimalised. Metal has memory, but when it's heated it stretches as it cools it shrinks causing metal to loose it's memory, when you use a hammer and dolly your giving your metal a reminder as to where it used to be.
I hope this helps.