"Those testing it: you may find it helpful to keep some screenshots with timestamps, for later reference."
That would seem to point towards an action that appears in the chat window...
"Can confirm only that (a) it is not just an empty perk, and (b) it doesn't do anything that would lead top-end players to consider it mandatory for skilling, bossing or PvP.
PvM/Slayer not mentioned... Hmm...
"I'm not going to tell you the in-game effect, but I can tell you that when it triggers (and it triggers under the same conditions as any other perk) it rolls a number between 0 and 19 and compares that to the rank you have the perk at."
Like, say... just for example... getting a roll on the rare drop table?