They fit in between the Rx and speed controller, and also monitor the battery voltage, either the whole pack voltage, or each cell via the balance connector. When they detect the battery/cell voltage is too low, they ignore your throttle input and send a 'neutral' signal to the ESC instead to stop the motor and prevent the battery discharging further.
I've seen them often listed in online shops that do electric planes/helis as well as cars.
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