I was wondering same when I first read about them.
Then I found them at Woolworths supermarket when I was doing groceries, (and not very expensive I thought).
They are a type of plastic container, available in a range of sizes, although all seem to have the same size lid, (or at least the ones I saw)
The lid has a flexible seal that fits inside the container, and you can press the button, or levers, in the lid to help release the seal for removing the lid.
Upshot is, that because most of the lid fits about 2.5 cm down inside, it allows you to generate pressure inside if you push it in quickly,
which must come out some where,..... the chute.
In the process it will blow out most of the grounds sitting in the chute.
I found it works best if you open up the grind setting by about 20 or so notches.
Hope this helps.