Final answer:
A driver's hand extended downward typically indicates the intention to stop or slow down, serving as a non-mechanical signal recognized in traffic regulations.
Step-by-step explanation:
When a driver's hand is extended downward, it generally means they are signaling an intention to stop or decrease speed. This hand signal is recognized by road traffic regulations in many areas as a substitute for mechanical signals, especially in the event that the vehicle's brake lights are not working properly. To signal a stop, the driver extends their arm straight out of the window, and rotates it so that the hand is downward with the palm facing backward.