The system asks testers to reveal their emotional response to an advert, as it is playing, by clicking on the relevant small graphic of a human head on their screen.
I think it was said earlier in this thread that the PPM is not meant to get into the details of response or reaction to the content. Just that the content was received. Tracking response is much more accurate and valid when done in a controlled environment, where the respondents are not distracted by other things, and can focus on the advertisement itself.