The opposite of bulge(). The middle of the picture is sucked inward, as though the image were being pulled down a drain.
It is the same operation with the sign flipped: each pixel's distance from the centre goes through a power, but this time distance stretches at the edges and compresses in the middle, so everything is drawn toward the centre point.
One thing to notice is that a pinch is strongest at middle distances and does nothing at all at the exact centre or at the very edge. That is what produces a funnel shape rather than a uniform shrink, and it is why the effect looks like something being drawn into a hole rather than the picture simply getting smaller.
strength sets how deep the funnel is, aspectLens whether it is circular or follows the frame, and wrap decides what fills the border that the inward pull leaves empty.