Klem -
I suspect a "poke" rather than a gradual drift could possibly introduce more slack because the top of the d-loop would be accelerating up and back when the desired situation is maximum tension between the tip and the wedge. A drift backward is often added to extend stroke length for distance but there should be a sense of tension from the backcast (d-loop) as it travels backward at all times, even if it's much lighter than in a no drift situation.
Suggestion #1 - video
Dana has shown us how useful video can be. It would be incredibly useful if you had a video clip of said tail, you might even solve it yourself if you could see it from a ways back.
Sug. #2 - V-loop verses D-loop
Another way to check slack in the d-loop is to try a v-loop, which I suspect you may have already tried. By creating a flatter, "pointy" more energized d-loop in a slightly shallower angle to the water you might be able to detect parts of the d-loop that have slack, irregular shapes, etc - if there are any. The more aggressively formed d-loop is not likely to have slack. Setting the anchor becomes more of the issue with a v-loop, but when you get it right and then throttle back down to a d-loop again your loop should be nice and tight.
Sug. #3 - Study the timing of the d-loop without a forward cast
It might also help to stop at the key position and just watch to make sure there is enough of what I refer to as "opposing tension" in the d-loop between the top leg and the bottom leg & anchor. Do that a few times, then take the forward cast making sure you are tight all-around from tip to anchor at the key firing position.
One-armed D-loop trick:
I recently thought of a trick for the class I gave with Broadside this spring. I wanted an easy way to communicate D-loop mechanics, and it worked pretty well:
Take the bottom hand off the rod and put the butt on your midsection somewhere near the middle of your body. Now with only one hand on the rod, do a slow lift and sweep around to form a d-loop. Do not use the bottom hand, do not let the rod butt leave the body until the speed-up-and-stop stroke. No forward cast, just stop and watch what the d-loop does based on body and upper hand movement.
Here's what I found:
1) One-arm d-loops force people to use your body rotation to force momentum into the d-loop while showing you exactly how far you need to rotate the body to cast in any particular direction. I've found that the over-zealous use of the bottom hand for creating the d-loop is often the cause of turbulence in the d-loop. This also eliminates the common problem of "trunking" or kicking out the bottom hand like an elephant's trunk (you can't trunk with one arm)
2) Since there is no forward cast, the one-arm d-loop puts the whole focus is on making and watching that d-loop form and not coming forward at all. This is like having your single-hand students make only back casts for a minute to emphasize the importance, etc. Most importantly it reinforces the timing of the d-loop action building up to the anchor's kiss.
3) One arm D-loops emphasize how little effort it takes to create a perfect d-loop when the body and upper arm do the right thing. It emphasizes how little the bottom hand does for traditional d-loops (underhand techniques may differ).
Practice this until you are forming a beautiful d-loop with the one hand up and the butt against the body, no forward casts. Then put the bottom hand back on and keep it's function minimalized until the forward cast. Don't start your (smooth) acceleration until the point you remember the d-loop to be fully formed in the one-arm exercise. This should result in an effortless, fully formed and slack-less d-loop.
You might try shorter line / minimum effort casts made very clean and extend the line length a bit at a time, trying to keep the same clean stroke getting longer and longer.
Sometimes it can be just as effective to work on a cure than to diagnose the problem. After being inspired by a chat with Tim Rajeff at GGACC I have been working on a stroke that gets the top leg of the loop on a forward spey cast to be as straight as possible, a "javelin on a rope". I had been very bottom leg focused all these years, trying to get that to be perfectly straight off the rod tip to the wedge. But getting the top leg straight feels like throwing a javelin and sometimes when I am lucky enough to hit it perfectly the cast flies through the air like a lightning bolt with no turbulence nor even the hint of a tail because the top leg is so tight. I hope to master this stroke eventually, with a tight bottom leg as well and figure out how to do it every cast. Still workin' on it :wink: