  We observe an inverted crossover resonance in $\pi$-driven four-level
systems, where $F'-F=0,+1$. The signal is observed through saturated absorption
spectroscopy of the $(6s^{2})$ $^{1}S_{0}$ $-$ $(6s6p)$ $^{3}P_{1}$ transition
in $^{171}$Yb, where the nuclear spin $I=1/2$. The enhanced absorption signal
is used to generate a dispersive curve for 556 nm laser frequency stabilisation
and the stabilised light cools $^{171}$Yb atoms in a two-stage magneto-optical
trap, achieving temperatures of 20 $\mu$K. The Doppler-free spectroscopy scheme
is further used to measure isotopic frequency shifts and hyperfine separations
for the intercombination line in ytterbium.
