You may be referring to a new surgical method of cartilage repair called microfracture technique. The drilling of tiny holes through the cartilage and into the bone releases blood from the bone marrow. The blood forms clots in the holes and creates a scaffold for healing cartilage to build upon.
Research so far shows that putting weight on the healing cartilage delays healing. Loading the joint must be avoided for up to two months to foster maturation of the fibrous tissue.
On the other hand, motion of the healing cartilage is very important. Patients are put on a machine called continuous passive motion (CPM). The leg is strapped into this device and it automatically bends and straightens the knee over and over.
It appears that this kind of constant motion helps improve nutrition and metabolic activity at the cellular level. Somehow the CPM helps the knee form the right kind of cartilage. Hyaline cartilage is needed for a good, strong repair. Too much fibrous cartilage leaves the defect soft and more likely to fail.
It's likely that no serious damage will occur if you forget and put your leg down once or twice. For the best results, follow your surgeon's advice carefully.