When you're on top of the world...
Bar clearance is all about how efficiently you use the height given to by your takeoff. If your center of mass clears the bar by a foot (0.3m) and you knock the bar off, then your bar clearance is not very efficient. On the other hand, if your center of mass just reaches the height of the bar and you still clear it, then your bar clearance is very efficient indeed.
The Measure of Efficiency
How do you determine if your bar clearance is efficient? The easiest way to tell is to view the jump with your eye at the level of the bar and looking down the length of the bar.
Watch as the jumper passes over the bar and observe how much clearance there is between the top of the bar and the underside of the jumper. The less variation in clearance during the jumper's passage, the more efficient the clearance is. There will always be some variation in clearance because the jumper has lumps and bulges on the underside, but the lowest points on each of the jumper's downward bulges should pass equally close to the bar.
This view of what constitutes the most efficient bar clearance is hard to prove. At least I haven't seen a proof of it yet. The best evidence for it lies in the way articulated bodies (bodies with a rigid structure and joints) behave when they are off the ground. High jumpers are articulated bodies, so from this point forward I will just refer to jumpers.
Imagine that a high jumper is aboard the international space station in orbit around the earth; a zero-g environment. Now imagine that the jumper is in the exercise room, not touching anything, and "standing" at attention with her center of mass at the center of the room.
If she "arches her back" in such a way that her body forms a "uniform" curve from head to foot. What happens to her hips, feet and head with respect to her center of mass? her head and feet both move backward, and her hips move forward relative to her center of mass. The center of mass remains stationary.