Looks to me like ABC is trying to solve the DVR/Tivo conundrum. By flashing the tiny promo just before flipping back to the show, this means that even if viewers FF through the other promos, they have to see the flash in order to catch the very beginning of the show as it comes back from commercial break.
If this works for them, we might start seeing more of this in the future. I'm sure this will spark a whole new set of discussions over just exactly what constitutes "subliminal messaging", because I believe imike24 is correct that out and out subliminal advertising has been banned by the FCC. ABC will argue that the flash promo is not subliminal because it's obvious and long enough to enter the viewers consciousness. So, if this continues, the could open a new can of worms for networks and advertisers.