IMO, link juices are coming from the site who have higher PR
in order to share some organic juices to a site who has a smaller value of rankings. Example, if you are going to place a link to a good PR
sites that is related to your niche you will get a link share from it, let say you place your link to siteA and that siteA has a value of 10 and there are 5 links who is link to it. It means that all 5 site who is linking to siteA have a 2:10 ratio in getting a link juice.