Jam a tight slightly tapered plug into a case mouth held in a tight fitting ring.  Then, pull the plug out.  It will drag the brass with it a bit, lengthening the case and thinning the brass in the process.  Repeat for more stretching.  A slow and tedious, but effective way to lengthen a case a few thousandths of an inch if it's slightly short for the chamber.

Another technique that may be called the same is as follows:

Press the case into a tight ring.  Spin the case in the lathe, and use a rounded form tool inside the case to press the brass against the ring.  Slowly withdraw the form tool, forming and lengthing the brass.  Similarly, press the case onto a close fitting arbor, and spin it in the lathe.  Use a rounded button form tool on the outside of the case to flow brass toward the mouth, as you spin the mandrel/case in the lathe.

Here's a couple of pics of tooling I've made for nibbing.  The first was a die and a small ball silver soldered to a rod that I used for lengthening cases.  Not my favorite way.  The second pic is using a mandrel, a smooth roller in a knurling holder.  This method works a little better.  With quite a bit of experimenting, developing  a method and then following it, a little blood sweat and tears and you can make brass grow a quarter to half an inch.  Do this to a dozen cases and you'll ask yourself why???
GT

Click on the following link then page down to "Lengthening Brass with a Lathe"

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Wayne

I used to have a Shiloh in 40/90 straight. BACO sold cases
At 3.25” long and we’re headstamped 405 basic. There where made
The way Greg is doing it , but all the case mouths varied in thickness. Some very thin and lots of cases separations. At 4.00 each , I had Shiloh rebaerrel it
To 45-90.