My guess would be that the "crown" of the barrel is somehow better formed in the good barrels. I don't know if that's the correct term in English.
Of course you can get more power by snapping your wrist! Straight power, that is, not really anything different. It's easier to visualize if you imagine your arm as a whip. Some people have no hand to arm coordination and do not properly time the snap to benefit maximally. A lot of stuff is going on in a pool stroke, there is also hand strength which can give your stroke an extra boost by propelling the cue forward at the exact apex of the snap/swing. There is nothing magical about this, but the human body was not meant to generate power by swinging the forearm with a locked elbow like a robot. More muscle groups, plus timing, gives extra speed. When I see these locked joint gurus, I'm also reminded of karate:
The opposite would be locked elbow, locked wrist, with which you have difficulty getting any power at all. Also some people misallign their bodies or mistime the snap, so they get a downward motion. In some circumstances you can get more draw or spin this way. (Masse/pique).