The stripped down servo (Futaba FP-S 148).
The potentiometer (brass squared shaft) is used to find the null (stopped) point of the motor, when the transmitter is operated the motor works in both directions and alters speed with the TX stick movement.
GIVING A VERY COMPACT MOTOR AND SPEED CONTROL UNIT.
As the potentiometer null point is very sensitive, this can also be fine tuned using the TX servo adjustments.
Connection between the motor and propeller shaft is by using thick walled plastic sleeving (such as the covering from electric cable)