The basic turn signal circuit is pretty simple. Power is fed via the ignition or accessory switch to one side of the turn signal flasher unit, whether mechanical or electronic. The other side of the flasher is wired to the center of the turn signal switch, which may be thought of as Left - Off - Right. In Off, the circuit stops there. All the turn signal lamps on the Left side are wired to the Left side of the switch, and same for Right. When the switch is set to, say, Left, the circuit runs from battery through flasher through through switch to Left lights (in parallel) to battery return. The flasher simply interrupts the circuit every so often to make the lights flash.

The flasher relay is usually rated for a certain current. Less (one light burns out) often causes it to stop flashing; more (the extra light on the boat trailer) often causes it to flash too fast. So called "heavy duty" units usually don't care how many lamps they flash. Replacing incandescent lamps with LEDs drastically reduces the required current and may cause flashers to work incorrectly.