When youtube was flash based that is how flash would save it's cache file for the stream. to save a video you would go looking for what file descriptors were in

When youtube was flash based that is how flash would save it's cache file for the stream. to save a video you would go looking for what file descriptors were in use and yank them out of I believe /dev/fd and back into the filesystem. On windows the file was visible but locked behind the normal windows file locking shenanigans. To copy locked files on windows I used a program called hobocopy that utilized volume shadow tricks to work around the locks.