It's talking about the sensor on one of the motors:
The 'optical sensor whhel' I was talking about is that little black wheel on the motors shaft.
I assume your issue is that the wheel off-centers during the game, right? It's caused by the optical wheel developing a micro crack due to sudden changes of tempeature thus starting to slide on the shaft and giving invalid inputs to the sensor.
The method in the tutorial is making a small noth in both the wheel and the shaft and then filling it with epoxy. That forms a mechanical bond and holds the wheel tightly on the shaft.