Video feedback. Point a camera at the screen it is filming and you get an infinite tunnel of receding copies; this does the same thing, but with a transformation applied each time round.
Every frame, the previous output is scaled, rotated, bent, twisted and hue-shifted, then blended back in with whatever is coming in fresh, and stored for the next go round. Because the transformation happens on every pass, its effect stacks up. Scale a fraction above one and you get the tunnel. Add a small rotation and it becomes a spiral. Do both and you get the classic vortex.
Which of the nineteen blend modes you pick decides how the trail behaves as it recedes. Under screen it glows brighter going in; under multiply it darkens into a hole. The mixing is balanced so that feeding live input into the loop cannot run away and blow out to white.