While waiting at a traffic signal for the light to change from red to green, you notice that the left turn arrow changes green without the forward light changing green. Just by seeing the light change you step on the gas alittle bit and then realize that your lane is still stopped. Causing the car to flinch. Typically implies that you are alert or that you are in a rush to go somewhere. Senario may be applied vice versa.
(While second in line waiting at a traffic signal in the forward lane)
Driver: I really hope the light changes soon.
Passenger: Yeah me too.
~Left turn lane changes green, but forward light remains red~
~ Car first in line on forward lane begins to go as if the forward light was green, realizes it is still red and stops; causing the vehicle to flinch moving forward a few inches~
Driver: Whoa! Did you see that guy ahead of us!?
Passenger: Yeah! He just did a Signal Flinch!
Driver: He must be in a rush to get out of here.