I can't explain that unless there is a separate note with the removable fixed coupler. Here is the instructions:
Fail Condition:
When removing the rear coupler to attach the dummy coupler you will lose operational control of the front coupler on the model. There have also been a few reports of damage to the front coupler drive FET on the engine board.
Corrective Action:
After removing the rear electrically operate Proto-Coupler, install a ¼ watt, 4.7k ohm resistor in the two-pin Molex location where the coupler attached. This makes the system believe there is a rear coil coupler and the front coupler will operate normally. Additionally, the resistor is strong enough to handle any inadvertent rear coupler commands sent to it.