I've had a look at the source and it's pretty simple and fast but could even be a little faster:

float zProj = pow(abs(dot(input.Normal, float3(0.f, 1.f, 0.f))), ProjectionPower);
is obviously the same as
float zProj = pow(abs(input.Normal.y), ProjectionPower);
which can be shortened to
float zProj = input.Normal.y*input.Normal.y;
as ProjectionPower = 2 apparently gives the best results (the same goes for zProjx/y, of course).