I generally cringe when hear someone say to do this or SEE someone doing it if it is not done in a correct manner.If you bounce the cue off your hand and don't hear any strange sounds or rattles the cue is most likely fine.
You are correct for the most part of being able to diagnose a possible unwanted noise by doing this if......you know what you're doing. Not saying you don't know, but I have seen with my own eyes others that don't.
Without trying to get all scientific or anything, common sense should tell you.
The basic design of a cue is for the force (vibration) from striking the ball to travel down the entire length of the cue should be goal of the builder. ("should be" IMO) Not a lateral force in the middle of the cue butt even tho there may be some of that force on certain shots made with the cue.
Now if you lightly tap the nose of the cue butt with the palm of one hand while holding the cue towards the bottom between your thumb and a finger in the "sweet spot" you'll get a similar vibration that travels along the butt of the cue after striking a ball with the cue and possibly detect something that may be loose somewhere in the cue. This could be anything from the glue used not holding up on a joint pin, improper glues used during construction, construction prep before gluing, construction technique all the way back to something as simple as a loose weight bolt. This is always one of the first places to check if you have a "bad vibration: noise. Not the Beach Boys.. Good Vibration

The problem is, and the cringing comes when I have seen someone SMACK the cue hard with the palm because tapping it softly didn't produce any sounds that may be considered an unwanted noise and they wanted to hear a noise darn it!
Also, rotate the cue butt 90 degrees 3 times and do the light tap method in all 4 places of the 360 circle. I have seen some cues that would make a weird noise at zero degrees but not at 90 when it's rotated. Also try it with the shaft attached to the butt still tapping in the middle at the joint area with the shaft snugged down.
Now don't everyone run out and start smacking your cue looking for a noise that isn't there!
