When passing a car or turning a corner, sources familiar with the matter say drivers trigger the function by hitting the turn signal button instead of allowing a car to complete the maneuver without human intervention. Not only does this signal to the car it can complete the turn, but by manually triggering the move, this indicates the driver has given thought to the maneuver and believes it is safe. A small tweak with big implications. By passing on control of such maneuvers to the driver, once the move is activated responsibility also shifts from car system to driver — as pressing the button signals intent.