Orange slider is stationary.

Grey, blue and pink bars have revolution joints with red pin. Yellow conrods have the same length.

Green slider is fixed to blue bar. Its position can be adjusted. Violet slider is fixed to grey bar. Its position can be adjusted. Input: pink bar.

If distances from green and violet sliders to the red pin are equal, angles of blue and grey bars with pink bar are always equal.

The video was made based on suggestion of an YouTube user, wjf213.