  A population of cool dwarfs with extreme rotational velocities (v sini > 100
km/sec) is present in young open clusters. ROSAT observations have shown that
these very fast rotators exhibit a level of X-ray activity a factor of 3-5
below the saturated level which is usually observed (both in X-rays and other
magnetic activity indicators) for `normally' fast rotators. This phenomenon has
been denominated "Supersaturation". W UMa contact binaries seem to be
supersaturated as well, while the scatter in the rotation-activity relation for
RS CVn and BY Dra binaries does not allow us to clearly discern whether they
exhibit supersaturation or not. Supersaturation is not seen in Halpha for Alpha
Per supersaturated stars. Two alternative lines of interpretation are
discussed.
