Salt is the key here. Water will move from an area of less salt to more salt (more water to less water), and so when the potato is placed in the salt water, all the water that is inside the potato (yes, plants have a lot of water inside of them, that’s what gives a plant it’s structure) moves out by osmosis. Thus, the potato gets all flimsy and not crisp anymore. Much like if you were to water all your houseplants with salt water. They would all get flimsy, and then die, and then your parents would be upset so don’t try that at home please.