The part that has been missed so far is that its not just about the bars and hands/arm, but to greater extent about the feet and weight placement. The body is a kinetic chain, follow along:
Whether you chose to weight the front of the bike or not (your style) everything needs to start off in a neutral and balanced position (that means a balanced core) with your weight on your legs (your legs support your body mass, supply balance, and really are what should be powering and controlling the bike) and off of your arms (your arms are great at small yet precise movements--manipulations of bike--but crappy at balance and power (compared to legs), and they don't work well when they are weighted down).
To provide this position--a balance and stable core--you basically have to be half-way through a squat and half-way through a push-up. Now you are able to react, pro-act, etc. Any decent motocross school and/or decent MTB skills instruction should teach this (this is also body movement 101).
Bars too far forward (too low--away from you) means giving up the bend in your arms (range of motion) and your weight will end up tossed (bucked) forward onto your hands (if your arms are already straight, and your front wheel has to drop a foot, your core is going forward, too). Bad news. Too high and your arms will be all cramped up/bunched up in your chest and you won't be able to manipulate the bike effectively, thus you'll start to lean back to to retain a better position and then you'll be too far off the back. (Muscles function the best when the muscle belly is half-way contracted--thus, half push-up, half squat is most effective athletic position for the human body)
So where's this (half push-up, half squat) put your bars? That's the proper height. And, obviously, that can't be perfect all the time because your core should have a dynamic relationship with your bike: supper-duper steeps, they may be a bit too low; a fairly flat and flowy track, now they may be a bit too high. Nothing wrong with making adjustments depending on where you ride, and the difference between even 5 mm can really be felt if you're in tune with your rig. 15, 20 mm's is a huge difference.
Go watch some vid of the top guys--DH or moto-- and this is exactly what you'll see (Gwin and Dungey, great examples).
Really, it doesn't START at the bars, but more like where do the bars END UP when you're in good position on the bike.