The throttle of an engine or ESC controls how much power the motor generates. Opening the throttle allows more air and fuel (or electricity) into the engine, delivering more power to its output shaft. In a helicopter, opening the throttle needs to be correlated with collective pitch inputs (and to a lesser degree with cyclic and anti-torque control inputs), so that as the load on the rotor system is increased, the head speed is maintained.
Often throttle is used to mean the throttle channel of the radio system, and sometimes it is used interchangeably with the collective pitch control, as the one control on the transmitter controls both channels via the pitch curve and throttle curve.
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