This feature takes a 'snapshot' of the visible+selected F-Curves, and displays these in the background as 'ghosts curves' in the background. Such curves are drawn semi-transparent, slightly darker, and with dotted lines. To use, simply click the 'curve' button beside the Auto-Snapping selector. To clear, simply click that button again (with a different icon now). These 'ghost curves' are stored per Graph Editor instance, and are not saved to file (i.e. per session only). They are useful to be used as guides when refining the shape of existing curves.