Fail safes are a must for me. I wouldn't play without it. What the failsafe does is return the throtel servo to full brakes if:
- you run out of charge in your batteries
- if by any chance your throtel servo konks out &
- if someone switches on with the same frequency without asking.
Many a times I been in the 3rd senario and there was one time I crashed into a crub and my battery holder came off. It's not fun chasing after a car going at full speed!

Another way to avoid this is to attach a return spring to the servo horn. I run a Kyosho super ten and I attached the end to the spring to the bulk head brace so now if the by any chance the fail safe konks out on me i still have a return spring to bring my car to a complete stop.

regards.