The slump stretch is one of those exercises that work remarkably well for clearing up calf cramps, even though it's not logically clear why.
What I mean is that while you should feel a stretch in your thoracic spine - in fact, for me that's where I feel the biggest stretch, especially in the morning - the nerve that innervates the calf muscle joins the spine at S1 and S2, your sacral spine.
You'll also feel a stretch in your calf muscles, but it doesn't appear to be the muscle stretch that is relieving the cramp because all the calf stretches I have ever done have done nothing in this regard. This is why I think it has something to do with getting the nervous system.
The bottom line is that the slump stretch works to relieve calf cramps for me and a number of my clients. Because the basis of coaching is finding what interventions work and using those to get the best result, this is a stretch I heartily recommend.
I failed to mention that I had some very bad episodes of cramp during my athletic career, some that lasted days and affected mostly my calves, glutes and quads. I became very sensitive to when I was on the verge of an attack of cramp. Since using this stretch, I've had no symptoms at all.