My conclusion, chatting briefly with admin here I
Post# of 96879
Everyone can like / dislike a post once including their own. A email is sent to that user indicating if you liked or disliked their post. Most importantly a post shows on the board how many likes / dislikes it has. If a post has 1 like 1 dislike it will show as liked by default. So if someone dislikes your post 5 times all it needs is 5 likes to over turn it.
I think they nailed this imo, think about it, transparency at its best.