What's the difference between dry and moist heat in a heating pad?

Moist heat is widely reported by reviewers and chronic-pain communities to penetrate deeper into muscles, which many find more effective for back and menstrual pain. Many electric pads in this roundup offer a moist option via a dampened sponge or cover, while dedicated moist-heat units like the ThermoRelief generate moisture automatically. The tradeoff is that dry heat is simpler and lower-maintenance.