  A new class of transient, which has been hypothesized to accompany the
explosion of an aspherical compact supernova, would arise when streams of
ejecta collide outside the star. However, conditions that favour the prompt
release of radiation from the collision, such as a diffuse stellar envelope,
disfavour the creation of non-radial ejecta in the first place. To determine
whether the collision can both occur and be visible, we simulate aspherical
explosions using the HUJI-RICH moving-mesh hydrodynamics code and analyze them
in terms of diffusion measures defined for individual fluid elements. While our
simulations are highly idealized, they connect to realistic explosions via a
single dimensionless parameter. Defining two measures of the importance of
diffusivity (two versions of the inverse P\'eclet number), we find that one
varies in a way that indicates colliding ejecta can release a photon flash,
while the other does not. Examining the x-ray transient XT 080109 associated
with supernova SN 2008D, we find that its fluence and duration are consistent
with the properties of an ejecta collision in the aspherical model that is most
likely to emit a flash. Our results give tentative evidence for the possibility
of collision-induced flashes for a narrow and radius-dependent range of
asphericity, and motivate future radiation hydrodynamics simulations.
