I have often wondered how different it is to diffuse light through a surface versus bouncing off a surface — and I'm probably not the only one who wonders.
This test shows that diffusing light through a surface does look different than bouncing light off it. Many photo umbrellas can be used either way, as can light discs, white sheets and shower curtains. So we tried all of these, in pairs, bouncing or shining through each substance. And look at panels 5 and 6 — see how different the color is when I bounced light off of a shower curtain versus diffusing the light through it. You should ALWAYS assume that any white material will give you weird color, whether you bounce light off or diffuse through — hence you test!
Bottom Line: Diffusing through a surface gets a different result than bouncing, but it all depends on the materials and how you set things up.
BEWARE! These results only mean one thing: you will likely get a different look shining light through than from bouncing — but almost certainly not in the way seen here. Why not? Because the color and translucence qualities of plastic and fabric vary wildly, so what happened for me probably won't happen for you. Also, my lights were both about 4 feet from the white surface for all 8 shots, regardless of whether I was shooting off or through.
So if you put the lights closer to the white surface the light will get more contrasty — harder. That will be true whether you bounce or diffuse through.