You adjust the height that the die screws into the press.
When you fire a new case, the shoulder pushes forward a bit (.002-.007" or so-- depends on specific setup and headspace), and the walls push out a bit (increase diameter). If you bottom your die out to hit the ram/case-holder on your press, then you are resizing the case almost all the way back to "new" size. Each cycle, the case expands, then is resized back to new. That's stretching the brass forward and out, then squishing it back down to size. It will cause the brass to flow forward, and you case walls will grow thinner and longer (and you have to trim your cases). Eventually they will rupture because they're so thin.
The trick is to measure the headspace change (how far forward the shoulder moves) between new brass and fired brass. Then you slowly, incrementally spin the entire die into the press, checking the headspace of the fired case each time you adjust, until the headspace becomes .001-.002" less than when it was fired. This minimizes the resizing, and thus, the expansion that happens between each cycle. The brass will get thinner at a slower rate than a complete FL size, but the .001-.002" set-back is enough to allow reliable fit/function without sticky-bolts or having to beat things with a hammer. You get greatly increased brass life, to boot.
