From BEASTX Wiki
The tail rotor turns around instantly when doing backwards flying.
- Tail gyro gain too low. Increase tail gyro gain as described ->here and make sure you're using the tail gyro in HeadingLock mode (using Normal-Rate mode it is nearly impossible to perform backwards maneuvers!).
- No sufficient thrust produced by the tail rotor. Check tail pitch angles. Reduce the maximum amount of available tail pitch throw at Setup menu point E to prevent the tail blades from stalling or increase the tail pitch angle if it's too small. Use different (larger) tail rotor blades or increase the rotor rpm.