PT via Similar Parallelograms

Creation of this resource was inspired by this amazing volume of resources created by Steve Phelps. Below, we have parallelograms built off the sides of a right triangle. These parallelograms are similar to each other. You can alter the size of the "lower left angle" of each parallelogram via the purple slider. LARGE POINTS are moveable anywhere AT ANY TIME. How would you explain the phenomenon you see here in your own words?

Quick (Silent) Demo