A special plastic wrap is used that it breathes out, which allows some of the moisture to escape when baked in the microwave oven. Microwave bags for other vegetables would probably work fine, too. Trapping the moisture in just steams the potatoes, but some people prefer it this way. Try this some time: Buy the loose potatoes, and wash and pierce them with a fork a few times to keep them from bursting. Wrap a paper towel loosely around each potato and microwave. The towel absorbs some of the moisture when it bakes.