Essentially, when you go down into a Shadow Lair elevator, you get the pinwheel like you would at the end of any arcade level. However, instead of crowns, materials or lockboxes, this pinwheel has themes. Themes for the boss levels you're about to go down into.

For example, if you go into Snarbolax's shadow lair, you could roll poison for the currently existing shadow lair, or fire for a Vog Cub-themed level, ice for a Skolver-themed level (this is assuming that a Skolver is to a Vog Cub what a Frostifur is to an Ash Tail, based on the armour names)
Equally, going down into the Roarmulus Twins' shadow lair, you could roll fire for what is again the current lair, or you could roll poison for a ghostbusters-esque set of levels, you get the idea.

Naturally, it's not possible to roll the default theme (No double-fire Vanaduke, or double-shock Roarmulus), and it isn't possible to roll sleep, either.
I can't really decide if Stun should be rollable or not. I can imagine that'd get annoying fast, but unlike sleep it's possible to get stunned on a regular basis in normal levels, while sleep relies on a rare monster.