Birds can perform low-speed maneuvers at post-stall angles of attack (AoAs), owing in part to covert feathers—a set of self-actuating feathers located on the upper surface of the wings. During unsteady flow separation at large AoAs, these feathers protrude into the flow and provide lift enhancements, for reasons that are still not fully understood. To facilitate the use of covertfeather-inspired designs in bio-inspired aerial vehicles, and to enable plausible hypotheses for the utility of these feathers in biological flight, we investigate a model system in which a passively deployable, torsionally hinged flap is mounted on the suction surface of a stationary airfoil at a Reynolds number of (formula presented) = 1,000. We perform high-fidelity nonlinear simulations to quantify the effect of flap moment of inertia, torsional stiffness, and chordwise location on aerodynamic performance. We identify parameter values that provide lift improvements as high as 27% relative to the baseline flap-less case. Torsional stiffness is found to dictate the mean deflection angle of the flap, and the rotational inertia is demonstrated to determine the time dependent dynamics about that position. Behavioral regimes that categorize the dynamics of the flow-airfoil-flap system are provided using a k-means clustering algorithm from two meaningfully chosen length scales. The dominant physical mechanisms responsible for delivering significant aerodynamic benefits characteristic to these regimes are identified and a qualitative comparison between these regimes is performed.