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@futurebird @ptmesis @lienrag I just plotted it, and as far as I can figure out, if you stay inside the radius of the outer circle, the "cardioid" is a drop-shape, but if you move the pen a little slower at the start and end, it should be pretty circular. I think this would be a natural consequence of drawing on paper given the friction between pen and paper.
The orange line is linear in time (steps of angle), and the green line is plotted with just the radial component of the orange line.
@ptmesis @futurebird @lienrag I think it would be a cardioid if r was outside the first circle, but again, not sure.
Radial geometry always messes with my head 
@futurebird @lienrag Ok, it's definitely off-centre, looking at the looped video. But the offset to the centre seems to determine the radius of the inner circle.
@futurebird @lienrag Yeah, that's the big "what" moment for me as well. I think all you need is to draw a straight line through the centre, but it would have to be timed with the rotation speed. Otherwise you'd end up with a spiral.