In my view, a 'good balance' means a gun mounts evenly and smoothly.
When not balanced well, either the barrels tend to rise first, or the stock, both needing small but concious correction.
I can't say it more succinctly than that, but being superfluous hasn't stopped me before. Watch an accomplished shooter mount from the low-gun position and swing on a target. If the mount starts with a muzzle-dip or you detect any see-saw motion in the swing as he closes on the target...he's not as accomplished as he could be.
If you can see the muzzle rise on a smooth, continuous line from start to finish you can be assured that the shooter has a practiced mount and both hands are moving in concert, making the shot seem almost effortless....especially if he happens to execute the shot almost immediately after the mount is complete (not always necessary or desireable, but it doesn't happen by accident). When you've witnessed it, you'll know that the shooter has mastered the gun mount, and it was greatly facilitated by a well-balanced (for him) gun.
