Yup, I bought those after watching Susan Khalje's Craftsy couse some years ago
The pins are intended and recommended for matching seams (and stripes/plaids for garment sewers). You put the particular seam or stripe that you want to match in between the two prongs of the pin so that pin firmly holds the fabric at the particular point better than a single pin.
So, I don't know if the fork pins actually work better than the regular pins when it comes to pinning pattern pieces to slippery fabric. I'm a weight-and-rotary-cutter person so rarely pin patterns to fabric! However, because the end of the pins (the U-shaped part) are bent upward, I can see it is easier to push it in through the fabric on a flat surface like when you pin pattern pieces to fabric.