Shift4Me has three main components:
A sensor that reads the pedalling cadence, some electronics and a servo motor.
"1" A simple magnetic sensor that is mounted near the pedals.
"2" The electronics (an Arduino) that read the sensor data and control the servo
"3" The servo motor that pulls the derailleur cable
This button can act as a "bypass" to temporaily disable the shifting of gears as well as a "reset" to put you back in starting gear.