This reminded me to post another of Slate's great videos, which uses tracking across demographic groups as a speech is given.
Or rather, in these videos, as a selection from a speech is given. Or a selection from a speech from a selection of speeches ...
What they're using is a market research tool called a 'worm'. According to Wikipedia this was first developed in Australia and first employed in the 1998 Federal election.
Each member of the audience firstly fills out a questionnaire, used to describe the composition of the audience, and then each member has a dial, to which they select their feelings towards the vision or stimuli. This dial is checked centrally three times per second, and as the audience reacts differently over time, the collective feelings of the audience are gathered.The major Australian election debates happen on commercial TV, and the 'worm' was first introduced for commercial reasons - to spice it up. But it's been very controversial because the sample sizes are so small and in the last election John Howard was able to get it removed.