Supporting trunnion while pressing in a barrel.

While pressing in a barrel, when the trunnion is installed in the reciever, how is the trunnion supported? I saw a picture of a square bar thru the mag well and under the trunnion while the barrel was being pressed, but it looks like all the stress will be exerted on the bullet guide. Is there something resting on the bar that I can't see in the picture? I don't know how to post a pic, but I could e-mail it to someone who can.
Thanks.