If you look at pictures of actual periscopes, you will see a set pf "ridges" - which look like a cable welded on - spiraling up the periscope. These were supposed to disturb the wake caused by the periscope, thus making it a tad harder to spot - less disturbance of the surface. Not sure exactly how much they really worked - but it is one of those nit-picking details we all love to see on our boats. This mod adds them .....
Gonna look for a good pic and post it in a sec.
Actually, here is flakmonkey's thread releasing it - good ingame pic right at start:
http://www.subsim.com/radioroom//sho...d.php?t=119520