The stream runs on its own from the moment the page opens. The names at the top right open and close each panel: joining and leaving, interest, playback, and the chart. Every value can be typed directly or nudged with the marks on either side of the field. Panels can be dragged anywhere and snap to each other and to the edges of the screen, and double-clicking a panel title pins one panel to the top right instead.
The number of people watching at once is the difference between two flows. People arrive as a Poisson process, so many per minute on average, with the actual count for each moment drawn at random around it. People leave at a rate applied to each viewer, so the number leaving grows with the size of the crowd. That is why the count does not climb forever: it settles where the arrivals and the departures cancel, at roughly the joining rate divided by the leaving rate.
The leaving rate is the natural exit plus the boredom exit multiplied by one minus the interest. At full interest only the natural exit is left; at zero interest both apply in full. So interest never brings anyone in. It only holds on to the people who are already there. The joining rate is deliberately blind to it, because nobody can tell whether a stream is worth watching until they have opened it.
Interest itself is three sine waves of different periods laid on top of each other, plus a slow random drift. When a peak arrives the count does not jump with it. What happens first is that fewer people leave, and only once that difference has piled up for a while does the count visibly rise. That delay is exactly the gap you can see between the two lines on the chart.
The algorithm boost feeds the current size of the audience back into the joining rate, through a base-ten logarithm. A stream with an audience is pushed to more people than one without. Because it is a logarithm rather than a multiple, it does not snowball: ten times the viewers buys only one more step of boost. Set it to zero and the joining rate stops depending on the crowd at all.
Each dot on the screen is one viewer. Green is someone arriving, blue is someone watching, and red is someone on their way out. The bar at the top is the stream itself, and its brightness is the current interest. When the crowd grows past a few thousand the dots are thinned out for drawing, but the number in the corner is the count the model keeps, not the number of dots.