In isotropic etching all orientations or planes etch at the same rate, shapes evolve in a relatively simple fashion. For example a square hole would get rounded corners in isotropic etching. Anisotropic etchants have different rates for different orientations. This difference in rate can drastically change the evolving shape. Because of the differences in rates, some planes grow, while others disappear.
Holes are lower than the surface of the wafer and pegs are higher than the wafer.
Holes enlarge with time while pegs shrink. After long times, holes are dominated by slow planes, while pegs become dominated by fast planes. For example, an 'L' shaped hole has five concave corners and one convex corners. For concave corners, slow planes dominate.