This is not an actual flight mode, but rather, an important feature of ACRO and STABILIZE modes. Other flight control software refer to it as a flight mode, so for consistency, its included in the Copter flight modes documentation
What it is¶
Normally, in Copter, if arming is setup via the rudder stick, if throttle is lowered to idle in ACRO or STABILIZE modes, the stabilization is removed. While this prevents movement due to vibration while on the ground at idle throttle, it prevents stabilization in flight for aerobatic movements or rapid descents.
RCx_OPTION is enabled for switch ARM/DISARM (41) then stabilization at idle throttle is still fully active, which is often referred to as AIRMODE.