I called Steve's Archery in Walla Walla because I've seen Steve use them. They seem to work.
Steve says the infor you're looking for is in Bill Herzog's "Steelhead Drift Fishing" book, pg. 22.
1/2 - 3/4 inch dia foam. about an inch long. Use large sewing needle to thread yarn.
Steve says it used to be a guide secret among Seattle area guides. Personally, I don't believe in magic. I think guides and experienced fishermen just know how put something in front in front of fish, colors, and when to switch if what they're using doesn't work.
Maybe someday, I'll know. That's part of why I'm an Ifish member!