When a man is having sex with a woman who is having her period and he pulls out and slaps his penis against her stomach or thigh, leaving a red arrow shaped mark on her skin, which resembles an upvote arrow on Reddit. Signifies a job well done, or 'thumbs up'.
"She was having her period, but we banged anyways... When we were finished I gave her an upvote, she earned it!"
when you post a good post on a social platform (for instance reddit) and that post slowly gets upvoted, but when more people notice that particular post, your post will be upvoted faster and faster until it eventually reaches frontpage.
dude 1 :"oh shit, come over here bro! my post has been noticed by many people! my post gets upvoted faster and faster! it's like an upvote acceleration!"
dude 2:"nice bro, you'll hit the frontpage soon"!