According to the GIA a "knot" is "an included diamond crystal that extends to the surface of the diamond after fashioning"... A "twinning wisp" is a "series of pinpoints, clouds, or crystals that forms in a diamond's growth plane; associated with crystal distortion and twinning planes"... We just rejected a diamond that had a knot in it, but took the opportunity to add the picture to our inclusion library, here it is:
This is a close-up of the knot:
And for those who may be wondering, the plotting diagram for it looks like this with the knot indicated by the red arrow:
And the answer to Leonid's question is "we served the pics off of our server"