Deflection as it is commonly referred to in the pocket billiard world is when the cue ball moves slightly off the aim line in the opposite direction that it was stuck when using sidespin.
If you hit the cue ball on the right side which you must do from time to time physics demands that cue ball be pushed slightly to the left of the center line or intended shot line. So the real meaning of deflection is actually the cue ball being pushed or squirting off the aim line. That’s why the term deflection is now often referred to as cue ball squirt.
Most all cue sticks that are sold and marketed as Low Deflections cues are actually high deflection shafts but manufacturers don’t want to confuse the public by calling them high deflection shafts. Why they are actually high deflection shafts is because the shafts are lighter and more flexible so when they strike the side of the cue ball the shaft actually gives way more than a traditional shaft might. What that does is reduce the amount the cue ball is pushed off line in the opposite direction. So, low deflection shafts are actually shafts that deflect a lot 😉