Earlier quoted context omitted.
That's probably a very grey area heading towards subliminal advertising (at least, some lawyer somewhere could claim so). I'm not sure on the legal status of that sort of thing.
I'm noticing very short ads on YouTube (about 5sec) which are cheaper and more effective (ime) than the longer ones, which are usually skipped before they communicate the message and cause (more) annoyance.
1 second (and below) it might be possible to argue it's too short to register the advert consciously, making it a subliminal advert. Again I don't know if this is true and I'm completely making this up. It'd be interesting to find out the answer.