I have an old worn out Sonic Care brush head, with two magnets sticking
out the bottom.

The wife issued me a new design brush head that is differently shaped.
She breaks off the magnets for the refrigerator.

I decided to cut the plastic off and figure out how it works.
There are two magnets attached via a long piece of metal to the brush.
The long piece is welded via a pin to a U channel piece of metal.
The whole thing seems too stiff to move. I gave up.

I called up Dan and he said:
The battery in the handle is switched by two low sat transistors in push
pull on the primary of a transformer. The secondary is a capacitor that
forms a resonant tank with the inductance. The magnetic core is open and
couples through plastic to the two magnets on the brush head. The brush
head magnets are mounted on metal, that completes the magnetic loop. The
action on the two magnets is side to side. The magnets do not touch
anything, but vibrate. The pin in the brush head acts like a torsion
bar. The resonant frequency of the inductance and capacitor through the
turns, must be the same frequency as the resonate frequency of the brush
bar mass with the torsion bar. The U channel must be stiffly mounted to
the handle by tightening the big plastic nut. The electrical resonance
is high Q and so low loss. The mechanical resonance is high Q and so low
loss, but the cheek can dampen the side to side motion, and that is why
there is a plastic shroud approaching the brush.

The new brush head design does not need a shroud. It does not have a
stiff torsion bar pin, but rather a translational pivot point. This
allows the brush head to move rotationally instead of side to side. The
new brush then compresses into the tooth and brushes side to side with
each stroke.

What does it all mean? Dan is in demand for defibrillator design,
hydroplane carburetor design, amateur TV transmitter design, and a few
other things, but he seems to know how everything works, relative to my
primitive understanding. It always amazes me how I can pick the topics
and then he has so much depth on the topic.