Based on Betaflight 2.3.4
Enable feature MOTOR_STOP to make the new mode "IDLE ON" appear in modes tab of configurator.
If you set the range parallel to air mode then motors keep spinning at zero throttle while in air mode and motors stop when flying in normal mode.
Recommended to use with switch arming on a 3 position switch:
Pos 1: nothing (disarmed)
Pos 2: arm (take off and land - motors do not spin with zero throttle)
Pos 3: arm + air mode + idle on (flight - motors keep spinning with zero throttle)
See Boris' release notes here:
https://github.com/borisbstyle/betaflight/releases