I alway think of flatwings as big fish flys. My largest striped bass ever was on one and I have a lot of confidence in them
They have an undulating movement that I believe is very life-like
Mostly I tie 6-7'' ones that imitate large sand eels, but I have some smaller ones in my trout, bass, and saltwater fly boxes also
My flies are certainly not works of art like those of Mr. Abrams, but I think it's the action of these flies that make them fish so well.
I don't tie "named" flies and I leave off the junglecock, but take the tying style and adapt it to colors I like to use
Yellow over white, chartreuse over white, olive over yellow over white, and purple over yellow over white are my favorites

I tie a "muddler" style flat wing that I like alot too. The hydraulics of water passing over the deer hair head makes the tail wiggle in a most enticing manner