The information on this feature is linked in the January 2013 section of the readme under the title Extended Subtyping Constraint Combinations. The behavior is as expected: you can place a subset from a subtyping line to a role on a direct or indirect supertype, but not the other direction. Exclusion is also allowed, but not equality.
The reason this was not supported is because there is a better way to model it. A subset from the role to the subtype line would indicate that every instance of the supertype that plays that role is also an instance of the subtype. The appropriate way to model this is to make the role player for the constrained role the subtype instead of the supertype, which can be done by dragging the role box onto a shape for the subtype. As an equality constraint is equivalent to a subset constraint in each direction we also block the equality constraint.
The intent of the subset constraint was to allow a role to be optional on a supertype and mandatory on a subtype. The construct as currently implemented is much cleaner than creating a derived fact type and making it mandatory on the supertype.