"Hand in the Air" is a rule that was developed by Alexander Shulgin during his many... err... neurochemical adventures.
Shulgin, along with his merry band of psychonauts, very often congregrated together to try out some new compound that Shulgin had created. Now, when a group of people get together to try new psychoactives that no one has ever tried before, it is generally a good idea to lay out some ground rules.
One of the rules that was laid out was the "hand in the air" rule. Basically, what this rule means is that any sentence that is preceded by the person raising their hand in the air and saying "Hand in the air" is automatically assumed to be of deadly serious importance.
For example, suppose someone in the room says, "Hey, I smell smoke!" Now in a room full of tripping people, this can mean anything from "Can I get some of whatever it is that whoever is smoking?" to "I hear there are some forest fires in Northern California." Using this rule, however, if the person were to raise their hand in the air, and say, "Hand in the air, I smell smoke!" then it immediately becomes clear that the person is dead serious, and everyone needs to come back from whatever solar system or reality their drug-addled brains are currently inhabiting, and figure out what the fuck is on fire.
The other part of this rule is that you never, *EVER* abuse this rule. Ever. Not even if it would be really funny.