Why do instant photos need shaking?
The short answer: they do not need it, and shaking can make the picture worse. The original film manufacturer said so publicly for years, and the song that made the habit famous is not a developing instruction.
What is actually happening
When a print is pushed out of the camera, it passes between two rollers. Those rollers burst a pod of reagent at the edge of the frame and spread it in an even layer between the negative and the positive sheet. The image then develops inside a sealed sandwich. Nothing about that process is exposed to open air, so waving the print around does not add oxygen, and it does not add heat in any meaningful amount.
Why shaking can hurt
While the reagent is still fluid, the layers can shift against each other. Shake hard enough and you can produce blotches, streaks or separation where the chemistry has pooled unevenly. With older peel-apart film the risk was higher still.
What actually speeds it up
Temperature. The reaction is chemical, so it runs faster when warm and noticeably slower in the cold. Tucking a developing print into an inside pocket does more than any amount of waving. Keep it out of bright light for the first few seconds too, since the emulsion is still light-sensitive as it settles.
So why does everyone shake?
Because waiting is unbearable and your hands want a job. That is the honest answer, and it is also why the gesture survived long after the chemistry stopped justifying it.
The ritual, kept
Instant Film Maxx keeps the wait and lets you shake through it. Give the phone a shake and the image rises out of the emulsion over about three seconds. It is the one case where shaking genuinely does something, because here it is the gesture that matters rather than the chemistry.
Instant Film Maxx is an instant film camera for iPhone. Shoot, shake, and watch the picture come up. Get it on the App Store.