[Unclenick]

The only thing compressing does that is different from increasing % powder fill in a case that still has room left over is that it reduces the volume the powder starts burning in more rapidly per additional grain. This is because it consumes not only the room the additional powder needs, but subtracts it from space between the grains of the total charge at the same time. The result is that pressure grows more rapidly with charge increases above 100% fill.

That has a limit, since you could theoretically squeeze all the space between the grains out, making a solid with no way for the flame front to move through it. But for most rifle powders that solid state is somewhere between 160% to 180% filled, which is well beyond how much you can compress without the case bulging or the neck buckling anyway.

Use a normal work up until you get to the compressed point, then go in small increments, like .3 grains or so, while watching for pressure signs, and you will be perfectly safe.

High percent compressing lets you use somewhat slower or bulkier powder than you might normally choose for a bullet.