  We study recent changes on the star formation history (SFH) of galaxies by
means of the ageing diagram (AD), tracing the fraction of stars formed during
the last $\sim 20$ Myr through the equivalent width of the H$\alpha$ line and
$\sim 1-3$ Gyr through the dust-corrected optical colour $(g-r)_0$ or the
Balmer break. We provide a physical characterization by using Pipe3D estimates
of the SFH of CALIFA and MaNGA galaxies, in combination with the predictions
from IllustrisTNG-100. Our results show that the AD may be divided into four
domains that correlate with the stellar mass fractions formed in the last 20
Myr and 3 Gyr: Ageing systems, whose SFR changes on scales of several Gyr,
account for $70-80\%$ of the galaxy population. Objects whose SFH was abruptly
truncated in the last $\sim 1$ Gyr arrange along a detached Quenched sequence
that represents $\sim 5-10\%$ by (volume-corrected) number for $10^9 < M_*/{\rm
M_\odot} < 10^{12}$. Undetermined systems represent an intermediate population
between the Ageing and Quenched regimes. Finally, Retired galaxies, dominated
by old stellar populations, are located at the region in the AD where the
Ageing and Quenched sequences converge. Defining different star formation
activity levels in terms of the birth rate parameter $b\equiv
\frac{SFR}{\langle SFR \rangle}$, we find that galaxies transit from the Ageing
to Quenched sequences on scales $\sim 500$ Myr. We conclude that the ageing
diagram provides a useful tool to discern recently Quenched galaxies from the
dominant Ageing population.
