Noose,
In a transfer system, the trailer box is transferred into the truck box which has rails for it inside. The truck box tailgate lifts up high enough so the trailer box fits underneath it. The transfer is done with air motors and the truck usually has to be level or on a slight downhill. The trailer box is locked inside the truck box with 2 big air operated hooks on the truck box that go over 2 pins on the trailer box once in place.
To make the transfer, the truck backs up to the trailer and locks a 2 pins in the trailer into 2 holes in the rear of the truck box, so everything is lined up for the transfer.
It is a good system for areas of limited access since only the truck needs to negotiate the access. A good operator can do the transfer in a couple of minutes to time isn't a real issue. In California, where the max GVW is 80,000 lbs., a transfer can carry 25 to 26 tons of material.
Hope this helps