In the ideal smile, the upper lip sits just at or above the top of the teeth. The general rule is that 1 to 2 millimeters of gum exposure with a maximal smile is aesthetically acceptable. When some people smile, however, a large gap becomes exposed between the lip and the upper teeth, exposing a lot of gum tissue and producing the so-called gummy smile. The gummy smile can be due to a variety of factors including a hyperactive muscle which lifts the upper lip too much, a thin upper lip which rolls under when smiling, gum tissue which covers too much of the upper teeth, and a long upper jaw.
Relaxing the hyperactive muscles of the upper lip which help lift it will reduce how much the upper lip moves upward resulting in less of a gummy smile.