The video (shown further down below) depicts lighting setups that are easily achieved. In this case, photographer Jyrki Vesa uses a Photoflex collapsible diffuser (a scrim) to simply and efficiently modify his twin-speedlight main-light rig. (Ya think Jyrki's name is pronounced like it looks? Jerky?)
Regardless, there's nothing jerky about Jerky's setups. They're simple, practical, and get the job done with little fuss-n-muss. (Keeping things simple being the notion I repeatedly beat readers over the head with in my Guerrilla Glamour ebook.)
I've shot a bunch of times using a simple, flat diffuser (a scrim) as a modifier. One of my scrims is attached to a shaft and opens like an umbrella. It's the one I most often use when modifying a source light with a flat diffuser. It doesn't matter, BTW, if you use small flashes, a monolight, or a head, you'll generally get the same results.
Like a reflective umbrella, a flat diffuser will spread the light over a wide area. Then why not use an umbrella? The difference, I've found, in using a flat scrim over an umbrella is the light spreads more evenly and flatly. I'm no physicist but I'm thinking the parabolic shape of an umbrella keeps the light more focused towards the center of wherever the umbrella is pointing. In other words, while umbrellas aren't the best choice when light control is your goal, and neither is a scrim, an umbrella is going to trump a flat diffuser in terms of light spread.
Here's the video. Enjoy.
The pretty girl at the top is Alexis. She's about to jump into the shower but graciously gave me a few minutes to set-up and photograph the experience. I lit Alexis with two light sources: My handy-dandy scrim-on-a-shaft for a mainlight and a small, shoot-thru umbrella, to the left, as a kicker.