  The second Gaia data release contains the identification of 147 535 low-mass
($\le 1.4 M_{\odot}$) rotational modulation variable candidates on (or close
to) the main sequence, together with their rotation period and modulation
amplitude. The richness, the period and amplitude range, and the photometric
precision of this sample make it possible to unveil, for the first time,
signatures of different surface inhomogeneity regimes in the amplitude-period
density diagram. The modulation amplitude distribution shows a clear
bimodality, with an evident gap at periods $P \le 2$ d. The low amplitude
branch, in turn, shows a period bimodality with a main clustering at periods $P
\approx$ 5 - 10 d and a secondary clustering of ultra-fast rotators at $P \le
0.5$ d. The amplitude-period multimodality is correlated with the position in
the period-absolute magnitude (or period-color) diagram, with the low- and
high-amplitude stars occupying different preferential locations. Here we argue
that such a multimodality represents a further evidence of the existence of
different regimes of surface inhomogeneities in young and middle-age low-mass
stars and we lay out possible scenarios for their evolution, which manifestly
include rapid transitions from one regime to another. In particular, the data
indicate that stars spinning up close to break-up velocity undergo a very rapid
change in their surface inhomogeneities configuration, which is revealed here
for the first time. The multimodality can be exploited to identify field stars
of age $\sim$ 100 -- 600 Myr belonging to the slow-rotator low-amplitude
sequence, for which age can be estimated from the rotation period via
gyrochronology relationships.
