Minecraft wiki entry for Development_versions, associated text: Super Secret Settings
The button now applies different experimental shaders.
List of available shaders: antialias, deconverge, outline, art, desaturate, pencil, bits, flip, phosphor, blobs, fxaa, scan_pincushion, blobs2, green, sobel, blur, invert, wobble, color_convolve, NTSC.