Just for starters, the rig I'm familiar with uses old soft ropes that are padded at the pastern. It's fastened to a surcingle around the horse's girth.
Of course, you use it when the horse is at a standstill in a round pen on soft bedding. "Running" doesn't refer to the horse running, but to the way the rope runs through the surcingle to give you enough leverage to raise the horse's foot. It makes more or less the shape of a "W".
Here's a commercial version, looks pretty much like what we used.
I have several books on horses in the movies and a trip wire and running w are 2 different things. The pic you posted is what they used in the movies except those lines that go towards the horses back end instead went to the riders hands. That way, at the operative moment, they jerked those lines and pulled the horses feet out from under them. Or they also tied the lines to a tree and the horses ran until they reached the end of the wires. The trip wire was a wire running between 2 points that the horse wouldn't see. I'll find my books and post the titles and authors if anybody wants to read them. There are some horrendous pics in them of the running w in action. Also, it explains in the book that the "running" in running w refers to how the wires are run, not the horse running.
By the way, I didn't mean that you were using something inhumane on your horses, I was just giving info on what a running w used to be used for.