Thanks for your great explanation @eroche. Now I understand I how it work. I tried to upvote after 30 minutes and made sure that I'm among the earliest upvoter for the posts that appear on 'hot' but earn nothing but I got some curation when I upvoted early on the post that only show a small pay out. Now I understand why... So I'll get nothing if I upvote a post that's already being upvoted by a whale or dolphin. Thanks again!