It's what I was taught: you switch on your indicators when you have checked that you are clear to merge and you have effectively committed. I always assume that someone who has put their indicators in is going to move according to them, whether it's clear or not.

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.

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

It is what I see in practice in Eastern Europe. They signal as they are shifting lanes. Even if they turn the blinker on and then start moving 1 second later, it could be considered the same thing as 1 second is negligible.

Thus "the indicator shows that you ARE moving." is correct, at least in practice.

It's the difference between actually purposefully blinking and blinking to avoid a fine. In the latter you just tap the blinker stalk as you're turning the wheel. If someone's trying to do a dangerousish turn (waiting for a line of cars to do an illegal U turn for example) they'll be blinking to signal intention most of the time.

I got my license in 2014, in Germany, and was taught to turn on the turn signal > check mirrors > turn your head to look over your shoulder and only then, when you're clear, do you merge.