RPM Brake allows you to set a different brake strength that works off of the motor RPM. This setting adjusts your “slow speed” or “infield brake” power. First you’ll set your normal Brake Strength for high speed areas of the track, so set them how you want them for braking on the straight and for corners at the end of high speed sections. Then you can use RPM brake to increase braking strength at slow speeds, if necessary.
If your car is perfect braking from high speeds, but seems to lack brake strength at slower speeds, like in the infield in tight sections, you can turn RPM Brake up to increase the slow speed brake power.