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When you fire a round the firing pin drives the round forward against the shoulder. Pressure rises and the case expands agsinst the chamber wall and the case stretches until it hits the bolt face. This is why brass lengthens and needs trimmed. After the pressure drops the brass cools and shrinks enough it can be extracted from the chamber and ejected. I use the hornady head space comparator on a caliper to measure the case length and can compare it to the prefired measurement. Then when you resize the fired brass set the die so it only pushes the shoulder back .002" to .007" (depending on application) from fired length as to keep from over working the brass and giving it excessive headspace when rechambered and fired.