Hall Effect Throttles:
A hall effect throttle is simply a magnet and a pickup sensor which works similarly to a potentiometer except rather than turning a manual dial, you move a magnet closer to a hall sensor.
In simple terms, when you connect a hall effect throttle to a 5 volt source, the sensor will detect the proximity of the magnet to the sensor itself. When the magnet is far away from the sensor, the output reading is 0 volts. As the magnet slowly gets closer to the sensor, the voltage gradually climbs higher. Usually there is a point where the magnet can get no closer to the sensor because of its protective housing, so you may only get from 0v to 4.6v from a hall effect rather than the 0-5V a potentiometer would give you. Most circuits take this into consideration, and you know when your circuit doesn't as you will never get full speed.