If it is slipped tendon, or even a dislocated femur, here is what I do (there may be better cures):
Hold the bird in your right hand by the shanks, being careful not to put any stress on the ankles, Place your left hand under the breast to support its weight, then lower your left hand to hang the bird by the shanks.
On a larger bird, gravity is usually sufficient to help stretch ligaments and tendons and allow the joint to relocate, but most chicks and juveniles don't weigh enough. You can encourage the chick to move the joint by rolling your wrist so that its breast is up, and most birds will flap and flex their legs and try to right themselves, frequently popping the offending part back into place.
Don't spend too long at this, and don't do it shortly after the chick has had food or drink to prevent crop regurgitation and possible aspiration.
Find the tendon on a mature bird first, to make it an easy anatomy lesson. It's the equivalent of our Achilles tendon, and can be felt on a grown bird just above the hock. It's tight when flexed, firm when straight.
My guess is a dislocated femur, because even with a slipped tendon the bird generally tries to use the leg. With a dislocated femur, they avoid using the leg at all.
