Typically done with longer shears, "back cutting" is a layering technique that's almost invisible to the eye but adds loads of movement to the hair. (Antonio said this cut works especially well on clients with long hair and medium density, or if you want to reduce any added weight on wigs.)
First, the stylist will divide the hair into top and bottom sections (the latter of which will be what gets cut). On the bottom section, pull the hair outward with your fingers and — instead of trimming the ends of the hair — cut starting at the root of the head downward toward the tips. Doing this to the bottom section ensures you're removing extra bulk that makes hair look or feel heavy, but without the obvious layers that tend to come with the territory.