I don't think this is what MOCAŠ was trying to say - his point, I believe, was that you should do a quick check to make sure there's nobody tearing down towards you in the outside lane and has put themselves in a situation where seeing your indicator flash would cause them to panic and do something drastic like brake too hard and lose control.
Thanks - that is basically what I was saying. To my mind, if a driver in the lane to my left puts his right-hand indicator on, he's saying that he intends to move into my lane. Now depending whereabouts my car is in relation to his, the balance of probabilities may suggest that he either is or is not aware of my presence. For instance, I would tend to assume that if I'm in his blind spot, there's a good chance he may not have seen me, and may therefore pull out anyway. I would therefore probably ease off or brake to allow him out, provided I wouldn't be adversely affecting anyone else by doing so.
This is what I mean by taking evasive action. Not necessarily anything as drastic as a swerve or an emergency stop, but perhaps an unintended lane change or braking.
Now, if I were the driver that was looking to make the manoeuvre, I wouldn't want someone in the next lane to feel they needed to take such evasive action, so I would look first to see if there was anyone approaching, or already alongside. If so, I would wait until they were out of my blind spot before indicating, so that they could be certain that I was aware of their presence. And, of course, I would only then indicate if there were no further vehicles to take account of.
Having indicated, I would then look to make the manoeuvre as swiftly as possible. I don't really hold with the idea of driving along with an indicator on when there's no opportunity to make a move, although there are probably one or two exceptions - for instance, in slow-moving traffic where I am as good as asking a driver to let me change lanes, but in that scenario, there would be no need for the other driver to take
evasive action.