It is taking on some of the characteristics of an animal or other creature while trancing or in the astral realm. It is an ancient practice and arguably a prehistoric practice. Mystics do it for many reasons, from blessing a hunt and thanking the game to taking on specific traits of an animal in order to utilize the characteristics and embody them. For example a witch might don a peacock costume with a fan and dance and move like the bird to a rythm that puts the witch into trance in order to gain the charisma of the peacock and add that power to their personality. Perhaps some people think it means literally changing shape into a different creature like Jacob in Twilight, but most practitioners I know describe it similarly to how I have here. Lupa has an article here that explains it well too. Halp Lupa where is your article about the ‘physics’. I learned a good deal of what I know from meet ups with Lupa, her books and articles are helpful too. Most the rest is from experience trial and error.
I do shapeshifting. A good half the time I wear a mask or costume of the animal. Some folks get like an animal hide, but honestly, I just made a dance costume from felt or used a halloween mask. I have danced into trance in the costume imitating the movement of the animal until I began to sense a change and I was soaring. Another time I used sex magic to totally trance out and shift into the animal and to embody the characteristics from the animal I wanted to posess. Another time I used deep breathing to go into trance and then push out my subtle body and shift into into the animal and then I pulled the subtle body back into myself. Then there are the many times I have then used such skills to shift astrally and in lucid dreams. Great topic.