Think of it this way, they grow under the shell. The shell itself doesn't grow. So they molt the shell away so they can expand, they do this quite fast, almost like a sponge. That's why you must be careful when a cray molts because they are extremely soft. I had one die from molting labor and i could barley get a hold of him because any pressure i put on his body just squished him in more. It takes about a day to regrow the shell depending on how big the cray is.
It's also important to keep other crays away from a fresh molted cray. I had a cray get in a fight with some guy who just molted, at the end of the day he had one claw, and 3 legs. The other cray actually got his claw UNDER the carapace (top part of shell) and pulled out three gills. The cray survived however, just barley though, i had to nurse him back to health in a separate tank.
But don't worry, and lost appendages will grow back with time, depending on the size of the cray it can take anywhere from 2-4 molts.
Hope that helps!
