It's two lines of dialogue. {nw} no-wait tag at the end of the first one, then the second line is identical except for the last word, and appears at instant speed. I dunno if the dialogue window would have flickered/changed to give it away but since the whole screen is black it didn't matter. I also had to disable the ability to change the text speed in preferences because it wouldn't have worked if everything was at instant speed.
Glad you thought it was a cool effect :D