Here is how I attempt to explain it to my bobber drifting brethren:
First off you pretty much need moving water. Nymphing in stillwater is a different game. The key is to stay tight enough to the fly (or split shot) that any interception of the fly immediately causes things to start tightening up. Think of each cast as "setting a trap". So that anything that picks up your fly immediately causes everything to start tightening up.
Don't concentrate on the end of your fly line, but instead concentrate on the ANGLE of the fly line between the tip of your rod and the water surface. Remember that you are an "ANGLER" (key word there). Do your best to maintain that angle as the fly drifts....and anytime that angle changes, and things begin to tighten up....SET THE HOOK. As long as you are doing your part correctly the only things that can cause the line to tighten up are #1. Your fly has bumped the bottom. #2. Your line or leader has drifted across an opposing current seam. #3. Your fly is in a fishes mouth.
So cast.
Then mend to "set the trap".
Then become an "angler"...in the true sense of the word. Follow the TRAP with your rod tip while maintaining a constant angle in the line between rod tip and water surface, and watch for something (uncontrollable) to change.
If you are bumping bottom too often, or not often enough....alter your weight or your tippet length.
If you are always getting swung into opposing currents....change your positioning, or get better at mending.
Now that THAT is out of the way, my favorite method for fishing dead slick water behind boulders is to sight-fish it. Use a fly with a hotspot, or something you can see....and set the hook when it suddenly disappears.