B Failsafe stands for Battery Failsafe.
It is available with PCM modulation,on the Futaba 9ZAP WC2 radio I have, when using 4,8V only , not 6V.
It is a safety feature that acts like I will describe below.
When you fly your plane, and you have enabled B Failsafe, then when voltage of the Receiver battery falls below 3,8 V and remains there for a few seconds, the Receiver recognizes that you are in a dangerous position and immediately brings your throttle servo to a preset position that you have set.
I set it at idle, so If I suddenly while I fly, I hear my engine to go to idle, then I know I have to land ASAP, because my RX battery is dying.
It is possible to regain full control on the throttle channel, by just pulling my throttle stick to idle and re advance it. This will give 30 seconds to land safely, before this feature reactivates.
This feature is not available if you use 6V battery, so I prefer to install stronger servos on my models , than using and stressing weaker on 6V.

I hope that I did not confused you more

Baper[8D]