rawlusA
Better description...
These adapters sense the brake light current (when you apply the brakes) and Pulse the rear brakes light in some set sequence...
Cheap units just make the brakes pulse as long a the foot is on the brake...and most of them are real annoying for us behind..
More sophisticated systems can detect how hard you want to brake and flash faster or slower depending on you foot braking conditions
Some will throw the pulses only for a few seconds as you slow, and once you stop they stay brake light on steady
I have not found any jurisdiction that objects to, or has any ordnances against these devices...
Most folks add to the center high mounted Brake light... but I have seen them on all the Brake lights
Some motorcycle brands have them as factory equipment DoT approved