A subframe transfers the load (and stress) to two points on the tractor - a receiver that attaches where your 3 point hitch arms are and a front receiver that replaces a heavy u shaped connector plate right about the middle of the tractor. The backhoe looks like an old cannon when it is unmounted. The subframe is analagous to a gun carriage - in this case an A frame of steel, and slides (carefully) into the receivers where a cross bar on the hoe frame is captured by the rear receiver and the front receiver has a long pin that locks it in.
Why go to the weight and expense (not to mention diminished ground clearance) of a sub-frame? Simple, both you and your tractor will end up living much longer. The sub-frame transfers the loads to the entire tractor frame - and you are talking about somthing powerfull enough to lift up and move your tractor. Search back through the archives and especially check the safety section. Backhoes, like aircraft, are terribly unforgiving of mistakes. Just imagine if the top link (or an arm connection) failed while you were sitting on a 3ph backhoe (hint - think squashed like a bug). Not only that, think of the stress on the tractor. When I have my stabilizers down and the bucket down I am in essence forming a 3 point contact with the ground. Stress is transferred to directly under the rear wheel and secondarily directly under the FEL mounting frame - so things are far less likely to break.
Lastly, start slow and build your technique. I started out on snow and ice piles (had a bit o' that this year) and worked on getting smooth action. The smoother you are the longer your equipment will last. Also, I think Harv had cited a book on backhoe technique (something I need to track down one of these days) - search and find that too. When I was looking, I sent an email to the Woods company (link on thier site) and got quick responses - which also specifically recommended sub-frame only for my tractor.
Good Luck!
Andy in NH