Eccentric step downs exercise--WHEN YOUR PT SAYS IT'S OK TO DO! My wife is a PT, so I can say that!
This has been a great (the best for me) rehab exercise after doing some damage to my R achilles. Eccentric movements are supposed to be very helpful for rehab in general.
I do these on any stairs--the higher the step the more difficult it is. Start with a short step. I do a set of 20 at a time per leg/side and it is a quad burner as well as being a great core exercise and a great balance exercise. It (as least for me) was much more difficult to do properly than I first thought it was.
You'll see that this guy is doing a couple of things. 1) He's doing the eccentric movement slowly to stay in that eccentric motion as long as reasonably possibly. 2) He's balancing on the ball (forefoot) of his foot that's up on the step. In other words, keep that heel down as much as possible close to the step without putting weight on it while balancing on that forefoot. Keeping the heel down there will stretch that achilles more during the exercise.
One thing I do to make it harder than the video shows is to touch my heel to the floor, not my toes, and reach out with that heel as much as you can. Especially, reaching out with that heel stretches that achilles out while it's in the rehabbing eccentric movement.
My guess is you're far from this point and I'm not a PT, so do what your PT says to do, but you might ask them about eccentric movement at the proper time and this exercise in particular.
Right now you'll probably just be working on basic range of motion with that ankle. Don't overdo it too quickly and get hurt again--not even a trophy elk is worth that.