Stencil bits are set wherever the shadow mask fails the depth test. That is, for all the pixels that land behind the platform, including its borders.

When the shadow is rendered, it is drawn wherever stencil bits are set. Normally, pixels that land behind the platform fail the depth test and aren't rendered.

But, at the borders, we also check against the pixels that are underneath the platform pixels (that is, the background pixels). Depth test passes there, so the shadow is rendered over the background pixels. (the shadowed background pixels are then blended with the platform pixels during the final antialiasing pass)