Look at a point on a sinusoidal curve (y=sin(x)) with a tangent line.
Then, look at a point on the negative of your sinusoidal curve (y=-sin(x)) that is a mirror image of your first point.
Then, notice that these mirror image points have mirror image tangent lines.
The app uses a physical model to find the largest area triangle whose vertices lie on a smooth closed curve (defined by the red control points).
The triangle defined by the tangent lines at the vertices is displayed, along with its medians.
What do you notice about the maximum?
The polar line is the locus of the intersections of tangent lines at the ends of chords of teh parabola through a fixed point.
Turns out to be conceptually important - not just a curiosity.