to 'cripple' a fly I trim all the hackle off the bottom ( belly side ) of the fly and trim the tail down to 3-4 barbs, so it rides in the water more like a parachute version, so the body of the fly is stuck in the film of the water, somewhat submerged. The other technique I've used is to only grease the wing so the butt end of the fly rides low in the water. I've found these work best in slower water and foam lines.
that's my version anyways. . .