What about a pool stick gives it high or low deflection, is it the curvature of the tip, like more curvature, more deflection, or what?
Not sure about the tip, but I do know that deflection has a lot to do with the shaft. Low deflection shafts will bend or give when hitting the cue ball. High deflection shafts will be more rigid. They wont bend as much when hitting the cue ball. https://www.youtube.com/watch?v=G1C_O8AO8yY
The above answer is correct, but the reason that one shaft moves out of the way of the cue ball after contact resulting in lower cue ball deflection vs a more rigid shaft not moving out of the way if the cue ball causing higher deflection is all about front end mass if the shaft itself. The lighter the front end of the shaft is, the less deflection the shaft has. Low deflection shafts typically have the core of the shaft hollowed out for the first 5 or 6 inches for example to make the front end of the shaft lighter. They also have a lighter weight and shorter ferrule. This also adds to making the front if the shaft lighter. A standard shaft is solid wood and has a regular ferrule on it. This makes the front end of the shaft heavier resulting in higher deflection. The butt portion of the cue has nothing to do with it at all. It's only the weight of the front of the shaft. So you can out a low deflection shaft on any cue and make it low deflection.