A feature of some Tx and Rx models that support PCM. Failsafe is used so that the servo's go to a predefined position if the signal is lost. In an airplane this can be to go to a low idle while putting the plane in a gentle turn, but in a helicopter it is not as useful since helicopters are naturally unstable there is no predefined setting to prevent a crash.

A locking device in the ON position eliminates accidental release.

In terms of relay technology, when power is lost, the relay contacts fall back to a default position.

Internal circuitry that monitors a system and shuts it down or alerts the operator in the event of a problem.

a great is idea, and should be a part of your radio gear