The unusual thing is something remarkable that stands out in each scene’s improvised world. Improvisers identify and exploit the unusual thing to get laughs. An unusual thing stands in contrast to the usual things.
The usual things are the unremarkable rules that make up the improvised world. They are normal, boring things that improvisers do to progress the scene. The more normal and boring they are, the more usual they are.
The unusual thing stands out by contrasting with the usual things. The more usual things there are, the more the unusual thing stands out. When an improviser prematurely identifies the unusual thing, the less it stands out and the less suitable it is to be exploited for laughs. An unusual thing without usual things is not unusual.
An unusual thing naturally arises while attempting to perform usual things because an improvised world is being created in real-time with mime, words and imagination. It is beyond the capacity of any improviser to create a normal improvised world about which there is nothing unusual.
Because more usual things strengthen the unusual thing, and because the unusual thing is an inevitable byproduct of doing usual things, the improviser should focus on, and put effort into, doing usual, normal, boring things. The unusual thing does not need to be forced, and should not be forced. It will reveal itself in contrast to the usual things.