Drain stands for expiration; PageRank Drain thus indicates the expiration of the PageRank(PR).
The PageRank is included in the ranking of the linked external website in the case of an outbound link, an external link or text link. The higher the PageRank of the web page, the stronger its weighting. However, if there are several outbound links on a web page, then the weighting of the PageRank is reduced to the number of outbound links.
Example: If a website has a PageRank of PR5 and there is only one outbound link, then this PR value will be fully included in the weighting of the inbound website. However, if the outbound website has ten outbound links, then the credited PR value of the ten inbound websites drops considerably.