This is the Twist 2.0 image distortion plug-in written by Peter Uray.
"[...] The twist maps the input image onto the output image by translating the original
pixels according to a 2d vector field which is determined by the selected distortion
function. In order to support the user's creativity some parameters (up to eight,
depending on the function type selected) can be adjusted, resulting in different
effects. The output of the twist plugin is a geometrically distorted image.
All this applies to layers as well as to images.
The effect of a selected parameter setting can be viewed interactively in the
preview image. [...]"
Seems that it is currently hard to find on the net; hopefully this will become better. Attached are the sources and binaries for Microsoft Windows.