This shows where he pressed the hub into the bearing, not the bearing into the trailing arm. There are washers under the closed end wrench and probably a suitable sized socket (that's what I used) on the backside to catch the lip of the bearing and keep it from being forced out the back side.
Here's the same process in my thread:
http://forums.pelicanparts.com/porsche-911-technical-forum/735959-71-coupe-project-4.html#post8269143