jit.gl.cornerpin allows for mapping of textures and matrices to an output window by repositioning the window corners - essentially a software keystoning mechanism. This can be useful in basic projection mapping, when projecting from odd angles, or when keystoning is inadequate.
When creating OpenGL scenes that need to be software keystoned before final output, you can render the scene to a jit.gl.node and send the entire thing into jit.gl.cornerpin before passing it on to the final render window. This simple patch by forum user DigitalFX does a good job of demonstrating this technique:
VPT (Video Projection Tool) is a free, multi-purpose video projection mapping tool by HC Gilje. http://hcgilje.wordpress.com/vpt/
(please list things that you believe to be errors or omissions from the existing refpage)