The device also can be used as a signal generator for electric speed controller (ESC), then you can test your motor system without using a transmitter and receiver.
3 modes to check 1-3 servos :
Manual mode: Turn the knob with different speed, check the reaction time
Neutral mode: Make the servo go back to the neutral point
Automatic "window wiper" mode: Make the servo swing like window wipers in the biggest angle.
Size: 36.5 x 31.0 x 22mm