Your turn signal works because there is a heating element built into the
signal flasher. When the turnsignal is turned on, a connection is made to
your lights via the flasher. Inside the flasher is a heating element which
heats up a bi-metallic strip attached to a set of points. The bimetallic
strip heats up causing the points to open, which shuts off the heating element.
The bimetallic strip cools, causing the points to close and the process begins
anew.

Ric