This is the awesome Venom Racing Fail Safe Unit.  The Fail Safe is mounted in line between the receiver and throttle servo.  It accurately monitors the signal received by your radio, and in the event of a glitch, the unit will return servo to a set position.  It features a built-in low voltage cut off at 4.2V.  It is very small, measuring 1.3" x .65" x .33".  It will fit nearly anywhere, and at this great price, it is a steal.  Why risk destroying your several hundred dollar car, truck, or boat when you don't have to even worry about it?  This is cheap insurance.