We found an amazing guide on curveballs on reddit (/r/thesilphroad), so decided to share it here also. Thanks to ohbearly for this great guide.

TL;DR: If you are spinning the ball counter-clockwise, the ball has to hit pokemon on the left side (left half of the screen). If you are spinning the ball clockwise, the ball has to hit pokemon on the right side (right half of the screen). Absolutely nothing else matters. That is all.

How to get Curveball Bonus every time

I’ve been using curveball all the time because I find it’s easier to hit pokemon (with the exception of really faraway ones). But I’ve never paid attention if I was actually getting the bonus, because it was just 10 XP. I’ve even seen posts here complaining about not getting the bonus when they should have, but who cares, it’s measly 10 XP.

Then I saw the article that claimed that the curveball on average adds 1.7 multiplier to the capture rate. That’s nothing to sneeze at, and I could definitely use that. So I started to pay attention to the curveball bonus, and to my unpleasant surprised, I noticed that I’m only getting it maybe quarter of the time.

So I spend a couple of hours today experimenting with different aspects, like where to start the throw (left corner, middle, right corner), when to lift the finger off the screen (after crossing the middle of the screen or before), whether the curve crosses the middle line dividing the screen in two halves, or not, and finally where the ball hits pokemon (left half of screen or right half of screen). To my surprise, the only thing that matters is where the ball hits pokemon: for the counter-clockwise spinning curveballs, it has to be the left side (left half of the screen), for clockwise spinning curveballs, it has to hit pokemon on the right side (right half of the screen). You can start throwing anywhere you want, left or right corner — it’s just easier to start on the same side of the screen where the ball has to land on the pokemon, but it won’t affect getting the curveball bonus if you start in the other corner, or the middle of the screen. The ball does not have to cross the middle line of the screen to get the bonus either.

So it turns out I was throwing the curveball completely wrong all this time. Maybe I’m a moron, but hopefully this little PSA will help someone else not be a moron. And it feels like the catch rate improved quite a bit for me, now that I’m getting nearly 100% success with curveballs.

EDIT: In case it wasn’t completely clear from my explanation above, the key in my analysis is that the only thing that matters is the side that you hit the pokemon on (to the left of the middle line dividing the screen into 2 halves, or to the right) and the correct side is determined by the direction of the pokeball spin. Not the starting point/corner, nor the trajectory, nor anything else.

TL;DR: If you are spinning the ball counter-clockwise, the ball has to hit pokemon on the left side (left half of the screen). If you are spinning the ball clockwise, the ball has to hit pokemon on the right side (right half of the screen). Absolutely nothing else matters. That is all.