From what I could tell, it was mucoid plaque. Hard, rubbery, and lumpy like the shape of my intestines, with little folds in it and lumps and thin parts. Later I passed more plaque, some ropes, some little bits and pieces. I'd guess I passed a total of 2 or 3 cups of the stuff (if I'd crammed it together). None of it looked remotely like a normal bowel movement.
I haven't passed any more since I stopped the shakes. If you're naturally passing this stuff, good for you! As they say, better out than in!