I did race Pro MX. Including a #1 plate in Nor Cal CMC MX and a #2 plate in AMA D36(Rodney Smith was #1) I do ride with former AMA National and World Champions in both Off Road and MX. I may have a #1 plate in AMA/D36 Cross Country and perhaps one or more AMA/D36 #1 plates in Enduro?
When watching a AMA National MX on TV. Concentrate on the riders knee position. It is NOT gripping a fixed location on the bike. If one was "always" gripping the bike with ones knees. The grip point would never change.
The basics; When accelerating the rider is in a more forward position, the exception would be slick conditions when the rider is trying to find more traction by getting some weight on the rear tire.
Braking the rider moves to the rear of the bike, as speed reduces one would move forward to weight the front tire, as you move thru the apex of the turn.
60-70 mph plus, one can lean forward into the wind and balance against the wind pressure.
The "balanced" position one should maintain changes depending on many factors, for example acceleration, braking, downhill, uphill...Many suggest that one should just lock on "all the time". I say BS. That grip point changes all the time. It is more a squeeze and release, to reset ones grip point.
You can do a quick and easy test right at your desk. Stand up, feet footpeg width. Your balanced, right? Now put your hands on your desk and lean on the desk. Feel the pressure on the palms of your hands? Your not balanced, your leaning on the desk. Now stand up again, bend forward, maintaining your balance, (core muscles should be used) now put your hands just above the desk or very lightly touching the desk. You should be able to lift you hands up and maintain balance. If you have solid railing or your truck bed, something you can pull on you can do the same test leaning back. Just grap the bar in front you and lean back. You should feel pressure on your fingers, like hanging from a pull up bar. Now stand up straight and move your hips back, as you bend your knees. Reach out like your going to grap the bar, but do NOT grap it. You should be in a squat position, "balanced" with your hands just litely touching the bar. No pressure on your fingers. Easy to move your fingers since you don't need them to hang on to the bar. What you are using is leg and core muscles to maintain your balance.
This is how one should hold the bars, a nice lite grip. If your out of position, not balanced on the bike. You must hold on with a death grip or squeeze the bike. Your getting worked. The guy who is balanced on bike maintains a nice lite grip and has his body weight in position to work/weight the pegs.
Think about a childs teeter totter. Your weight is over the fulcrum point, one foot on either side. The kids start going up and down. To avoid falling off as one side lifts and the other side, falls. You would have maintain your balance? If you stayed vertical in relation to the board, you would fall off. If you had a bar to hold onto, one could hang on. But the guy who stand vertical to the ground and allows his legs to work, up/down would be balanced and would not need something to hang on to. Think about that board being your footpegs, rising and falling, as you introduce right and left lean angles on your bike. Now look at a pro going around a flat turn, no berm or bank. His weight is to the outside. This allows the rider to weight the outside peg at maximum lean angle when the tires begin to slip out. If his weight was located over the inside peg and he lost grip. He would not be in position to weight the outside peg as the tires are sliding away from his body weight and down he would go.