Eliminate Shape Distortion at Origin
Selecting "Eliminate Shape Distortion at Origin" (found in the Options popup menu at the upper right of the Flex Projection panel) changes the height-to-width proportions of the projection such that shape is not distorted at the origin of the projection. The origin is the central point of the projection graticule.
The effect of this command can be easily seen when the Tissot indictarices are visible. After applying the command, the Tissot indicatrix at the center of the projection is a perfect circle, and not an ellipse.