These are great questions! From using ARC Pro and ARC Air, I am also assuming some form of dynamic braking - a small reverse current going to the motor when the trigger is released.
The brake button itself is an on/off switch, as in the previous Scalextric Sport Digital systems. However, from a peek in the communication protocols, it does look like there's scope for more varied braking responses - possibly with ARC compatible Slot.it controllers that have adjustable braking. It would be good if this switched off dynamic braking.
I do use the brake button with cars that I've removed the magnets from - just a very quick blip at the end of a straight. There's a sense that there is some dynamic braking with standard non-mag cars, but for cars with traction magnets and cars with high-torque motors (like the Sierra I built here) they do stop quite sharpish.
I find it a challenge to roll cars with magnets round my R1 hairpin - partly because they stop when I release the trigger and partly because there's little 'tail-out' drift - and I do find myself needing to keep some throttle through R2 esses. It's a different way of driving to the non-magnet cars. The Sierra really is tricky to drive, in part I need to learn to drive competely differently with the extra torque, but I presume my difficulties are compounded by the dynamic braking. Keeping a little throttle going does definitely help.
Malcolm - are you running cars with the traction magnets in them? And what cars are you using? Some are a bit more stuck down than others! For example, the BTCC Civics are rather more grippy than the MGs, VWs and BMW 125s as their traction magnets sit much closer to the rails on the track. All modern cars are very different beasts compared to the old Scalextric cars that had no magnets or much weaker ceramic magnets.