[Percula]

As long as you are HU, you can flip one or both pre-flop if you want to. Doesn't matter when or if the action is on you or not, literally anytime you are HU.

But be forwarned... It is not as big a +EV thing as some seem to think it is. When the guy looks "shocked" at seeing your hand, what does it mean when he looks "shocked". Is he shocked that you called with that "crap", is he shocked that he is drawing dead, is he just shocked you turned over your cards?

Your hand really has to be 180° from his read and be 180° of you image to get a useful reaction. And most good and/or experienced players are only likely to flash their true reaction for a micro-second and if you can not read it, well you just gave him a ton of information.

It can also be used against you and VERY effectively too. Here is an example... You have KK on the button and a guy MP limps (he covers you) and you raise to 5xBB and you and the limper in MP go HU to the flop. Flop comes K99 rainbow, he checks, you make a PSB and he calls. Turn comes a 3, he checks, you make a PSB and he calls. River is a 6 and he checks and you make a PSB and he RR 3xPot and now you flip over the KK to get a read and decide to raise, he instantly goes AI. Do you call knowing you have shown him the 2nd nuts?

I have only been able to use it for a solid read once and many years.