The calf stretch is important if the knee has been held in flexion for some time, perhaps after a long spell on crutches.
In case you cannot see the video, here is a still sequence -
|Stand opposite a wall, with your fingers supporting you on the wall.|
|Step back on the one leg. Lean forward, stretching the back of the rear leg's calf and ankle. Hold for 20 seconds. Repeat five times.|