[lbschenkel]

I don't doubt that it's the way you have been taught, but it doesn't make any sense. The whole point of blinkers/indicator lights in cars are to signal your intentions before you do them: if you're going to signal at the same time that you do the action you're signalling, you might as well not bother.

[rcxdude]

You signal in advance, but you check before you signal. Mirrors, signals, maneuver.