[kazinator]

According to the DIY build instructions, the bottles are cut in half, so they provide horn-like flares, open on both ends.

(Not that this adds any plausibility.)

The technical claim is summarized in this image:

http://i.dailymail.co.uk/i/pix/2016/06/10/18/3521DBE60000057...

[Nomentatus]

Compressing, then expanding air, so this is the same principle as a fridge uses with freon (ok, not freon any more), on a much gentler scale, with much-lesser-but-still-useful results.