These are giant hackles that flow from the base of the saddle and along side of the tail of a rooster. They are opaque and webby. They are great for very large saltwater streamers and have found much favor with steelhead fly tyers. Schlappen from Scottish roosters were the original spey fly hackle. For this application, strip one side of the feather and saturate the portion that you are going to use with saliva. The saliva will break the barbules down slightly so that they won't marry together and will remain separated, providing lots of action when fishing.
The magenta feather at the bottom of the picture is a normal 4" - 5" regular Chinese rooster saddle hackle. It is in the picture so that you can gauge the larger size of schlappen hackles above it.