Why does one of my ducklings keep twitching?

We put a duck egg under one of our broody hens. Yesterday I found it sort of crushed/attacked in the box but the duckling inside was still alive! I bought it in and we put it in a box on the back of our Aga. It was fine and then this morning it was out of the shell but still attached by a few small stringy bits to the membrane by like a lump in its tummy. What is this? It got loose but now it's twitching lots, especially its legs. Will it be ok? Does anyone know why or what this is? We've hatched chicks for a few years but never experienced this.