First, imagine a thin, springy, steel band, perhaps an
inch wide and several feet long. Further, holding it by
one end, and let the other end hang down. Now, shake
this band by the end you're holding. Traverse waves
will move down the band from top to bottom.
Next, put this imaginary band
inside of a square tube,
whose inner dimensions is one inch x one inch. Now,
when the top of the band is shaken, not only does the
band experience a traverse wave, but there are pockets
of air being moved down the tube by the surface of the
band.
Replace the one inch square tube with a different shape
enclosure -- two (opposing) sides are one inch
rectangles, the other two (opposing) sides are
trapezoids, which start out quite wide at the top, and
narrow down to one inch at the bottom.
When the steel band is shaken in this new shape, it still
forms a traverse wave moving downwards, but the
amplitude of this wave decreases as the wave moves
downwards in the enclosure, and the size of the air
pockets decreases as they are moved from the top of
the tube to the bottom.
Assuming that air isn't leaking between the metal band
and the enclosure, decreasing the size of the air
pockets must compress the air in those pockets.
What does this compressor have that others don't?
A: Gas moves in a unidirectional manner, which is thermodynamically beneficial.
B: It's very easy to provide intercooling.
C: The movement of the flat surface of the band along
the enclosure is more of a rolling motion than a sliding
one, so not only is there relatively little friction there,
but dirt particles are relatively unlikely to damage the
compressor.