The barrel has a narrow portion that is cut down just after the gas block and under the handguard retainer... The barrel press simply uses this larger shoulder on the barrel to support the barrel when you press. It is a simple design but works very well.
You can see it in this photo:
http://quarterbore.com/images/amdbui...pacing-08.html
and
http://quarterbore.com/images/amdbui...pacing-09.html
Both of those are from this tutorial that I have not found the time to finish yet...
http://quarterbore.com/images/amdbuild7/index.html