  Since their formation, stars slow down their rotation rates by the removal of
angular momentum from their surfaces, e.g. via stellar winds. Despite the
complexity of the processes involved, a traditional model, where the removal of
angular momentum loss by magnetic fields is prescribed, has provided a useful
framework to understand observational relations between stellar rotation and
age and magnetic field strength. Here, a spindown model is proposed where loss
of angular momentum by magnetic fields is evolved dynamically, instead of being
kinematically prescribed. To this end, we evolve the stellar rotation and
magnetic field simultaneously over stellar evolution time by extending our
previous work on a dynamo model which incorporates the nonlinear feedback
mechanisms on rotation and magnetic fields. Our extended model reproduces key
observations and explains the presence of the two branches of (fast and slow
rotating) stars which have different relations between rotation rate $\Omega$
vs. time (age), magnetic field strength $|B|$ vs. rotation rate, and frequency
of magnetic field $\omega_{cyc}$ vs. rotation rate. For fast rotating stars we
find: (i) an exponential spindown $\Omega \propto e^{-1.35t}$, with $t$
measured in Gyrs, (ii) $|B|$ saturates for higher rotation rate, (iii)
$\omega_{cyc} \propto \Omega^{0.85}$. For slow rotating stars we obtain: (i) a
power law spindown $\Omega \propto t^{-0.52}$, (ii) $|B|$ scales almost
linearly with rotation rate, (iii) $\omega_{cyc} \propto \Omega^{1.16}$. The
results obtained are in good agreement with observations. The Vaughan-Preston
gap is consistently explained in our model by the shortest spindown timescale
in this transition from fast to slow rotators. Our results highlight the
importance of self-regulation of magnetic fields and rotation by direct and
indirect interactions involving nonlinear feedback in stellar evolution.
