Cat flaps- how do ID disks work?

If it's what I think it is, it uses a RFID (radio frequency identification) chip in the tag along with a reader in the door frame. They are basically the same as those tags on clothes that set off an alarm when someone takes a garment out through the sensors. the tag opens a lock in the frame and allows the cat wearing the tag to open the flap. They do work and they are definitely worth the money.