The seat disk predates the Korky flapper, so your toilet must be really old if it has a flush valve that uses one. It’s a flat rubber disk that fits over the valve opening, held in place by water ...

The module runs on a set of four AA batteries, that should last for 6 to 12 months depending on how much action the toilet sees. And if users don't want the now-useless flush lever to still be hanging ...