Hi,
The Sphere_AO material function returns a fraction that tells you the fraction of occlusion for a given position in the world that is given by a sphere of a given size and location.
Put another way, its asking "How much does this Sphere occlude the Sky (or hemisphere). "
To use it on a flat blob shadow card you can simply plug in the location of your character and the sphere radius (or one per each leg etc). Because its a blob shadow you will have to balance between having the correct location and correct radius. Generally, using the Sphere_AO function you may want to boost the occlusion by either over estimating your sphere size or by applying a pow to the output perhaps if this is for a stylized game.
A more advanced/accurate way to use it on translucency is to use the “WorldPosition Behind Translucency” input as the position. I just look at the function though and for some reason I did not expose the Position as an input which is a pretty unfortunate oversight. If you go inside you can replace the “Absolute World Position” input with something else and use that and it will work. Or you could make it a material function and sample it on the ground of your environment but that wouldn’t be my first choice, doing is using translucency would be more reuable.