Try my best to explain. haha
Normally in pool, we will try to play alot of English aka sliding in order to put the cue ball into better position to pot the target ball.
Deflection sometimes called squirt, is the altering of the path of the cue ball when english is used.
Simply put it as, when you hit the cue ball on the right, the cue ball will travel to the left, and vice versa. So if you using English to play shots, you need to compensate the angle in order to shoot the ball straight in the true path.
The deflection level varies from cue to cue, also the table conditions, and type of balls used.
Some cue makers like the famous Predator, they created 10 spices laminated into one shaft instead of the traditional solid maple shaft, in order to minimize deflection.
To some players, this type of laminated shaft might be good, enable them to play shots easier but however to others, often this type of shaft felt much more hollower, lighter as compare to normal maple kind. This is also one of the reason why, many custom cue makers still using maple wood as their main choice to make a top premium shaft as they feel much more solid in the way they make contact.
Chance upon this video on deflection
http://www.youtube.com/watch?v=G1C_O8AO8yY from the way they address in this video, i think he is trying to promote predator shaft.