In the regular travels of a person, they might by some means form an attachment with some other world, leaving them permanently (or at least continually) connected with it. The potential for a Whimsy connection can occur one of two ways: firstly by a person actually traveling to a world (or past it in a red breach) or by visiting that place in dreams. In either case, once the connection has been made, thereafter a character tied to some world may experience a Whimsy.
A Whimsy occurs when a character interacts with some object or location that has some perceived tie to the world they are connected to. The location/object will give off a shimmer of sorts that the character and those nearby can sense along with the character gaining an awareness that the way is open to travel. The character may also briefly be aware that this is an opportunity which will be interesting with no downsides, though they will lose that certainly almost immediately afterwards leading the action of deciding to make use of it a 'flight of whimsy' which is where the name came from.
1. Any Whimsy is guaranteed to return the visitors to Infinitas eventually. In Whimsy-time, this could last any length of time. In Infinitas time, this will always be around a day and the character will return near where they left.
2. What happens in a Whimsy stays in a Whimsy. Any form changes, injuries, power boosts, etc will not last once Marsillon is returned to. Any items pulled back will be representative versions only. Ex: a mythical sword may end up as a bracelet charm. No matter how much time is spent in the Whimsy, anyone who departed will return as they left and only a day or so older. There may be residual effects however as mental effects in the form of phantom sensations that take some time to fade. If you have a form in the whimsy with a tail that you don't have outside the whimsy for instance, you'll keep feeling like you still have that tail for some time afterwards. Similarly if you lose an arm, then while you still have your arm outside the whimsy you're likely to be dealing with phantom pains and clumsiness with that limb for some time.
3. Whimsies have continuity in regards to the character form. If a Whimsy is engaged, then any characters will find themselves as they were when the previous Whimsy ended. If an item that was pulled back to Marsillon is brought with them, it will resume its usual form. If not, it is left in Marsillon as the representative form.
4. Whimsies are not exactly continuous in time. A Whimsy will always resume after the previous one ended, but typically not pick up exactly where the previous Whimsy left off. There is often a jump in time or location. This sometimes means that objects not brought back to Marsillon in representative form can get left behind.