A cross-section, seen from above: a bright window on the left, a curtain, a body, and the wall behind. With bare glass the sun is a small, hard source and the body throws a sharp-edged shadow. Add sheerness and the lit curtain itself becomes the source - the glowing emitter grows, and the shadow's edge melts into a soft gradient. Pulling the curtain in from the glass enlarges the lit patch of fabric too. Diffusion is nothing more than making the source bigger.