You put a cutter into the mouth of the case and cut a little bit of an angle on it. Allows the bullets to enter the case mouth easier. Also do the same to the outside of the case (with a different cutter) to clean off burrs and such after it is trimmed.
This is so you can see what it looks like.
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The Lyman is the 8th one down. The others are the combination inside outside chamfer and deburring tools. The difference is the angle they cut. The VLD is a steeper angle and allows easier starting. IMO