Its top surface is a connector which connects to the phone in place of the normal SIM. Its bottom surface is a connector which connects to the SIM in place of the phone. With electronics, it can modify signals in either direction, thus presenting a modified SIM to the phone, and/or presenting a modified phone to the SIM. It is a similar concept to the Game Genie, which connects between a game console and a game cartridge, creating a modified game.