Not to long ago I briefly fished a pool I had never had success in regardless of it's incredible fishiness. I realize, now having caught fish from it, how simple the solution was.
It is a pool on a nearby medium sized river, made up of four separate plunges into one cut out VERY deep trough. No gravel, just clean bedrock. It is essentially an extremely wide plunge pool. The amount of water coming down each fall is less the closer it is to the far bank. These are the important facts I did not know until recently: The upstream side of the pool is undercut by about a foot in most places, more bellow each plunge (not including the largest). The trout sit in the cuts facing downstream. There is also always a nice foam bunch adjacent to the nearest plunge, and their are typically fish sitting inches from the surface underneath it.
This was very good information, as it impacted my fly selection greatly. For the foam patch fish I used a weightless hairs ear. It punched through the foam and sat just underneath. I was essentially dapping it. The takes (all of which I missed) were like a classic mayfly take.
The undercut fish needed something bulky. A bead head hair's ear soft hackle sufficed. It needed to be cast into the plunges, where it was pushed down and in front of the trouts noses. The current would eventually drag the line and cause the fly to jump out of the cut. The strike detection was just like upstream indicator- free nymphing. At times I would pull the fly out quickly and a trout would come out chasing it frantically.
It felt good to finally decipher this spot.