Perhaps you are referring to body weight support treadmill training. This is where the patient is placed in a harness that is suspended above a treadmill. With assistance the patient is encouraged to step with the affected leg. Te suspension is adjusted so that the weak muscles can work without the full weight of gravity action on the leg. Likewise the speed of walking can be controlled to give the patient the best challenge they can manage. Here is a photo:

Figure 2

the technique has undergone considerable research with mixed results. My understanding is that more recent trials that emphasise a high intensity of practice can be helpful