  Multiple polarization observables must be measured to access the amplitude
structure of pseudoscalar meson photoproduction off the proton. The
hyperon-producing reactions are especially attractive to study, since the weak
decays allow straightforward measurement of the induced and recoil polarization
observables. In this paper we emphasize $\gamma + p \to K^+ + \Lambda$,
discussing recent measurements of $C_x$, $C_z$, and $P$ for this reaction. An
empirical constraint on the helicity amplitudes is obtained. A simplified model
involving spin-flip and spin non-flip amplitudes is presented. Finally, a
semi-classical model of how the polarization may arise is presented.
