  The special point is a feature unique to models of hybrid neutron stars. It
represents a location on their mass--radius sequences that is insensitive to
the phase transition density. We consider hybrid neutron stars with a core of
deconfined quark matter that obeys a constant--sound--speed (CSS) equation of
state model and provide a fit formula for the coordinates of the special point
as functions of the squared sound speed ($c_s^2$) and pressure scale ($A$)
parameters. Using the special point mass as a proxy for the maximum mass of the
hybrid stars we derive limits for the CSS model parameters based on the recent
NICER constraint on mass and radius of pulsar PSR J0740+6620, $0.36 <
{c^2_s}_{\rm min} < 0.43$ and $80<A [{\rm MeV/fm}^3]<160$. The upper limit for
the maximum mass of hybrid stars depends on the upper limit for $c_s^2$ so that
choosing $c^2_{s,max} = 0.6$ results in $M_{\rm max}<2.7~M_\odot$, within the
mass range of GW190814.
