FUTABA 6K Instruction Manual page 18

Self neutral type
A throttle stick returns
neutrally by a spring.
A throttle stick doesn't return
neutrally.
WARNING
You cannot use the throttle stick of self-neutral type for RC
airplane, RC helicopter, and certain multi-copter.
It's very dangerous if Engine / Motor becomes middle-speed
by self-return.
It's necessary to change the stick to the ratchet type if using it for
RC airplane and RC helicopter.
18
Throttle stick warning
（Multicopter/Robot specification）
Ratchet type（General transmitter）
Throttle stick：
Motor or engine
power is controlled.