  We experimentally demonstrate how magnetic depolarization of
velocity-selective optical pumping can be used to single out the collisional
cusp kernel best describing spin and velocity relaxing collisions between
potassium atoms and low pressure helium. The range of pressures and transverse
fields used simulate the novel optical pumping regime pertinent to sodium
guidestars employed in adaptive optics. We measure the precession of
spin-velocity modes under the application of transverse magnetic fields,
simulating the natural configuration of mesospheric sodium optical pumping in
the geomagnetic field. We also provide a full theoretical account of the
experimental data using the recently developed cusp kernels, which
realistically quantify velocity damping collisions in this novel optical
pumping regime. A single cusp kernel with a sharpness $s=13\pm 2$ provides a
global fit to the K-He data.
