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.