This is an old one. Brake force factor has been always a difficult parameter to understand and many newcomers or even advanced users tend to stay uninformed about it.
Rescued from 2010 this explanation from Frank Convertino appears very clear to me.
Ok, I am only going to speak of what I know about the in game brake force curve and will assume that its the same for the out of game adjustment.
First off the brake force curve has nothing to do with how sensitive the brake pedal will feel or react. What it does is depict how much of the in game brake will be applied via how much of the out of game input you are inputting via the pedal. In the sim there is only so much braking that is assigned to the throw of the pedal. The “curve” depicts where the majority of the force will be placed. So for instance if you have the curve set at .50 then the majority of the brake force will be placed with only 25% of the pedal being used. In other words 50% of the in sim force will be presented with only 25% of actual pedal input. Lets go the the other extreme, and put the curve to 5.0. With that setting the majority of the force will now be placed with close to maximum pedal input or closer to floored input. example will be 50% of the in sim braking will be accomplished with 80% pedal input and any more of that input would take up the remaining amount of braking potential with in the sim.
Lets put this to a real world example. I like using the H-pattern and driving cars that utilize the shifter. I also like to Heal/toe downshift. With my G-25 wheel set the pedals are flat across from each other. In other words the brake pedal is of the same height as the gas pedal. This is not found in real life cars, the brake pedal is a good 2 inches or more higher then the gas pedal, but under breaking it is about even with the gas pedal and therefor easier to reach when doing heal/toe downshifts. Not the car with my pedal set so in order to help counter this issue I run a very low brake curve (around .4-.5) setting in game. This give me about 60% brake force with very little pedal input and allows me to not have to move my foot to much in order to downshift. I’m hoping to change this with a mod that I have thought up and will looking into this weekend in which I’m going to raise the brake pedal. When I accomplish this I will most likely have to change the brake curve.
I hope this makes sense to you, if not I will try a different set of explanations. But remember, the world “curve” is being used literally here.
Remember to set it 0.0 if you are using a load cell based pedal.