How does the Ehiem impeller work?

Sometime ago, we purchased a tank which came with two Ehiem Model
2015(?) pumps and a third for spare parts. I've taken the head apart
and am examining the impeller, but don't understand how it works. This
is what I am seeing:There is a cylindrical magnet into which the
impeller shaft fits. The impeller snaps on to the top of this shaft and
can rotate maybe 180^ in either direction. Two ceramic rods run thru
the center of this assembly and serve to keep the assembly vertical.
I assume the external coil causes the cylindrical magnet to rotate.
However, there does not appear to be an direct connection between the
cyhlindrical magnet and the impeller shaft, and there does not appear
to be any magnetic interaction between the magnet and the shaft, hence
the impeller shaft only vibrates.
Can someone describe how the impeller is supposed to operate? - Mike