Actually, this referred to a tricky jump you had to make when you near the platform with the flower. Originally you had to climb to the middle of a ladder, then press jump, which would make the player back-flip unto the right platform. Because this was actually very hard to figure out – you never had to do anything like that before and the camera doesn't really show when to jump – we decided to put this rather cryptic sign there. Turns out though that a platform was moved a bit when the dungeon was decorated by the artists, so now you can also get there using a regular jump and I don't think anyone ever used the back-flip variant at all.
Hence the mystery of the sign.
