The trick is capable of freeing up gigabytes of space on your phone, which always come in handy if you're using a lower-capacity device.
To see the trick in action, go to Settings>General>About, and scroll down to 'Available'.
Check how much storage you have left, and then head to the iTunes Store.
Search for a film which is bigger than the space left on your phone. Something like The Lord of the Rings: The Two Towers (which clocks in at 6.84GB) should do the trick.
Then click 'Rent' at the top of the page. Don't worry - if you don't have room, you won't be charged, and an error box saying you don't have enough available storage will appear.
After you've clicked 'OK', head back to settings and check your available storage again. It should have increased, potentially quite significantly.
Before I tried this trick, I had 500MB of free space. After the first try, it increased to 848MB. It reached 1GB after the second try, and 1.3GB the third. In a minute of trying to rent the film, the phone gained 800MB of available storage space, a fairly impressive gain.
The trick was discussed on Reddit's iPhone forum, where some users with higher-capacity phones said they had gained up to 4GB of space in just a few tries.
It's not completely clear what causes the increase in available space, but it seems like the attempted download causes the phone to clear its caches.
This is a continuous process which is always happening on your phone, and the device will do it automatically when it's running low on space. This trick appears to force the phone to have a cleanout, freeing up a lot of space in one go.
We've contacted Apple to see if they can shed any light on the unusual space-saving solution.
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