I found the rejection bit tricky as well. What works for me is slightly swinging the yo-yo forward (away from you), whilst using your thumb to push the opposite way on the string (towards you)… you don’t want to swing too much, as it’ll make the slack miss the throw.
I’ve also found that the faster the throw is spinning, the better the rejection is. When it goes too slow for me, the slack doesn’t reject very well.
Hope that helps.