Generally there is a hydraulic "block" on the rear of the transmission. The backhoe hooks into that - there are some ports that have to be unscrewed and connect in hydraulic pipes. Generally you just need a single line in and out to provide pressure to the backhoe, the backhoe then has levers that have separate hydraulic valves all built in. I believe the connection is sometimes called the "power beyond" circuit, but that's probably the limit of my knowledge.