  Magneto-optical trapping forces for molecules are far weaker than for alkali
atoms because the photon scattering rate is reduced when there are multiple
ground states, and because of optical pumping into dark states. The force is
further reduced when the upper state has a much smaller Zeeman splitting than
the lower state. We use a rate model to estimate the strength of the trapping
and damping forces in a magneto-optical trap (MOT) of CaF molecules, using
either the A$^{2}\Pi_{1/2}$ - X$^{2}\Sigma^{+}$ transition or the
B$^{2}\Sigma^{+}$ - X$^{2}\Sigma^{+}$ transition. We identify a new mechanism
of magneto-optical trapping that arises when, in each beam of the MOT, two
laser components with opposite polarizations and different detunings address
the same transition. This mechanism produces a strong trapping force even when
the upper state has little or no Zeeman splitting. It is the main mechanism
responsible for the trapping force when the A$^{2}\Pi_{1/2}$ -
X$^{2}\Sigma^{+}$ transition is used.
