I first read about the EGW undersize carbide sizing dies at General Reloading - Brian Enos's Forums... Maku mozo! by competitive shooters using mixed range pickup brass.

The undersize die will compensate for thinner cases or older cases that have more brass spring back.

I use a Lyman type "M" die expander with my Lee undersize die to ensure straight inline bullet seating. Meaning the "M" die expander will keep the bullet from tilting during seating.

The Lee factory crimp die with the carbide ring in its base is a "cheat" for people who do not trim their brass. Meaning the longer cases will bulge below the crimp and the carbide ring will size this bulge.

If you are using cast bullets and the Lee FCD you should measure case diameter after bullet seating and again after using the Lee FCD. This is to check to see if the Lee FCD is reducing the cast bullet diameter that can effect accuracy.

Just remember you will have different thickness cases and its the thicker cases that will reduce the cast bullet diameter when using the Lee FCD.

Below a case wasp waisted case with very good bullet grip, and the reason behind the undersized dies and also showing the proper amount of taper crimp.