Actually, I think it's (red - (green + blue)). Equal parts red, green, blue, e.g. FFFFFF or 555555 is just gray. With this formula, that would be zero -- so no

Actually, I think it's (red - (green + blue)). Equal parts red, green, blue, e.g. FFFFFF or 555555 is just gray. With this formula, that would be zero -- so no red. With his formula, he's getting any pixel that has 100 out of 255 increments greater than the green and blue channels combined.