Its purpose is just to keep the same scheme/protocol. So http goes to http, https goes to https.
If the original URL was http and you receive a redirect "//forums.mirc.com/ubbthreads.php/forum_summary" the new effective url would be
http://forums.mirc.com/ubbthreads.php/forum_summaryIf the original URL was http
s with the same redirect "//forums.mirc.com/ubbthreads.php/forum_summary" the new effective url would be http
s://forums.mirc.com/ubbthreads.php/forum_summary
You can see this forum does it on a redirect, if you look at the source of
https://www.reddit.com you can see a bunch of resources use this format href="//
https://tools.ietf.org/html/rfc1808 URL = ( absoluteURL | relativeURL ) [ "#" fragment ]
absoluteURL = generic-RL | ( scheme ":" *( uchar | reserved ) )
generic-RL = scheme ":" relativeURL
relativeURL = net_path | abs_path | rel_path
net_path = "//" net_loc [ abs_path ]
abs_path = "/" rel_path
rel_path = [ path ] [ ";" params ] [ "?" query ]