Just to clear up EBD guys, it works exactly like when you brake a Mountain Bike with front and rear brakes. When stopping, you don't hit the rears as hard instantly, however when you wipe off some speed you can give the rears a little more squeeze. Stops the rear from locking up instantly when there's no weight over it.
Same works in a car. I learnt it (hence Danny's description) pulling up from a ridiculous speed over 100km/h to miss a roundabout in an 80 zone that suddenly appeared closer than I thought... needless to say the rear end lifted almost clean off the tarmac as the fronts gripped (down a hill too which made it worse). I felt the back end get really really light then almost instantly settle back down again and the car braked straight and true.
ABS stopped the lock up, but EBD in that instant would have dialled far more braking to the fronts when it realised there was almost no weight over the rears, then when I had wiped off some speed I was settled on all four wheels and continued braking solidly, sending more braking back to the rears again to stop the fronts from locking up as their weight lightened as the nose lifted from the initial shock. As much as a car has
ABS, EBD still works to prevent the
ABS being needed as early because a car under hard
ABS stop will actually pull up in a longer distance than one at maximum braking threshold before
ABS kicks in.
I've also got Stability and Traction control on my Zetec, which goes a step further and detects if the car is understeering or oversteering (measures 20 times per second constantly!) using
ABS sensors, a steering column angle sensor and a lateral-G-force (yaw) sensor under the passenger seat. If the car is oversteering, it applies maximum braking force to the wheel on the front outside corner to create drag and push the tail back in. For understeer, it brakes in the inside rear wheel to pull the nose back in by again creating drag. It combines this with cutting the power at appropriate levels to allow you to go around a corner even if you happen to be at full throttle when the slide develops. The car gradually allows power back until it senses that the full grip levels have returned, which may take a millisecond or a couple of seconds. It's a great system, allows you to have a little fun without cutting out everything and has saved my life at least once guaranteed so far, so well worth it.