  We determine the exact physical parameters of the four Hyades cluster K
giants, using their parallaxes and atmospheric modeling of our red-channel
TIGRE high-resolution spectra. Performing a comparison with well-tested
evolutionary tracks, we derive exact masses and evolutionary stages. At an age
of 588 ($\pm 60$) Myrs and with a metallicity of Z=0.03 (consistent with the
spectroscopic abundances), we find HD~27371 and HD~28307, the two less bright K
giants, at the onset of central helium-burning, entering their blue loops with
a mass of 2.62 $M_{\odot}$, while the slightly brighter stars HD~28305 and
HD~27697 are already exiting their blue loop. Their more advanced evolution
suggests a higher mass of 2.75 $M_{\odot}$.
  Notably, this pairing coincides with the different activity levels, which we
find for these four stars from chromospheric activity monitoring with TIGRE and
archival Mount Wilson data as well as from ROSAT coronal detections: The two
less evolved K giants are the far more active pair, and we confidently confirm
their rotation with periods of about 142 days. This work therefore provides
some first, direct evidence of magnetic braking during the 130~Myrs lasting
phase of central helium-burning, similar to what has long been known to occur
to cool main-sequence stars.
