So basically they have to have the app and when they have turned on notification in the settings, it pushes the notification to your device. And amazing too and really it is first time I know this, that people how much in terms of percentage they used in upvoting you. The notfication shows that instantly and I noticed that in the settings there is a slider for upvote percentage. Coincidentally too, just after getting on the app and turning notfication on, I started getting upvote on my other post of today that was stuck all day without upvote, I think it was 4upvotes in 8hours, now suddenly it is getting upvotes and I am getting notified and upon checking the post now, it has more than 40. I wonder what happened