"Invisible layers" is a simple technique used in haircutting that adds volume to a hairstyle without seeing "visible layering".  This technique is simple to learn and achieve.  Start by making rectangular sections on the top of the head, leaving the bang area out of the equation, if the hairstyle includes bangs.  Follow by making three equal subsections within the rectangular section, then take each subsection and tease from the scalp to the ends of the hair, making the subsections stand straight up.  Be sure that the teasing is really compacted from scalp to ends.  Continue teasing the other two subsections,then trim hair located in the third of each subsection.  Now carefully brush the teasing from each subsection and the invisible layering is complete!

NOTE:  If the client's hair is too thick and it's hard to get a good tease within the subsections, section the three subsections into smaller subsections for ease of teasing.