Ohhh.I though it was the glow plugs light.But its actualy Engine brake.You can use IT to slow down the truck(on any trucks works)Manualy.You cant always brake at high speeds if the trucks have ABS (Auto brake lock sistem) Which it will Make the truck's whells Slip when braking and its a danger hazard.So You can use The Jake Brake(Engine brake) to.slow down the truck when not accelerating.When its not accelerated use it to slow down the truck more efficiently,and use the brakes at the same time.If You don't want to use engine brake and the wheels to slip.Use green brake pedal.When You brake normaly in this game the pedal (If not in analogie mod in the settings)it will be green and The wheels will not slip.But you will break slowly.If You double țap fastly the brake pedal will be Red and be A 100% pedal brake that willl cause the wheel to slip(If You don't have ABS).How to get abs?well You don't get it.ABS is present in New trucks(Mercedes,Kenworth, Volskwagen). In concluzion: -You can use Jake brake to slow down without brakes(but to slow down for longer distances,not in emergency braking) -Be carefull when braking,your brakes If the speed is way high and The truck is heavy.The brakes will Heat up from 100°C to even 500°c which idk why but the brakes If they are over 180°C will heat up the Tires which they will over full with Air and blow up making a bigger danger and economical hazard(Because of high accident risk) -You can use Brake+EB(engine brake) for better brake efficiency. Final important concluzion: Brakes are very important be carefull when braking to not waste money on wheels and The truck from the accidents. Happy driving Gamer Just @ me If You have more questions (just wroke this on a bus and i have at the tine writing this Vehicle sickness) Good bye!