Mushroom or Bowl Cut
A: The "mushroom" cut you are referring to is also called a bowl cut, and features a weight-line that circumnavigates the perimeter of the head - usually at or just above the tops of the ears. The hair is usually combed smooth and parted in the center, or combed with no discernable parting (however is desired).
The hair below the weight line is usually cut into layers. One of the easiest methods of layering is to over-direct the hair below the weight line to the cutting point created by the weight line.
Each section of the hair is combed from beneath until the weight line is at a 90-degree elevation, and the hair beneath is angled upward to the same cutting point. The lower hairs are cut to the length of the weight line, creating smoothly spaced, deep layering.
The lower lengths of hair (below the weight line) can also be layered using a traditional method of creating a second guide length at the perimeter of the hair and working your way around the head from the center back to the front. The layers are formed by cutting the hair to line up the perimeter guide to the weight line at the necessary cutting angle.
©Hairfinder.com
See also:
How to cut a bowl cut for boys
Haircutting angles and elevations
Clipper cutting techniques
What are undershave and undercut haircuts?