The Undervoltage Cutback turns down the controller is the battery voltage falls too low. See Guided Features Tour for an explanation.
This operates at around 20v which is measured (as it must be) inside the controller. This means that measurement includes any drop in the battery wiring, so if the wiring is too long (or too thin) the available motor current will be reduced, as if the controller's current limit was set too low.
If you prefer not to remove the resistor, or find it easier, simply solder a resistor in parallel with the 33K. 100K will give an effective 25K and 68K gives an effective 22K.
The undervoltage protection level may be altered by changing one resistor value on the board: the resistor to be changed is arrowed in the diagram.
The resistor as fitted is 33K giving a cutoff voltage around 21v. If this resistor is reduced to 22K, the cutoff voltage reduced to around 16v, which is disabled to for all practical purposes.
It should be noted that this resistor is in series with the ignition wire to pin 2 of the connector. If this resistor is reduced, then the cutoff voltage can be trimmed by an external resistor in the ignition!
The only downside of reducing this too far is that the power relay might de-power if the voltage is too low. It is therefore sensible to chose a cutoff voltage above the relay drop-out voltage.
The system on the 48v version is identical - but the zener diode marked 3v9 on the diagram is replaced by an 8v2 zener. This is the zener two components to the right of the arrowed resistor. With the 8v2, the undervoltage comes in at about 38v.