Dan Blanton has chosen to empahasize one cause of a tailing loop and that cause is the rod tip traveling in a straight line at the end of the cast. But he doesn't go into much detail why this occurs.
It is true that the fly line needs to clear the rod tip to prevent a tailing loop. To accomplish that , you need to dip the rod tip down at the stop. This is what Doug Swisher causes the "micro wrist" in his two videos, Basic and Advanced Fly Casting.
The best casting site I know of is Sexy Loops, which is popular with competitive casters. Here is their explanation of tailing loops with a video that shows the concave rod path causing a tailing loop. Take a good look at the following two articles:
http://www.sexyloops.com/flycasting/tailingloops.shtml
http://www.sexyloops.com/flycasting/tailingloops2.shtml
Another recent article in Fly Fisherman Magazine by Jim McLennan's titled "The Creep & Jab" in the March 2008 issue gives one common cause. See a video and explanation here:
http://flyfisherman.com/videos/creepandjab/index.html
The "jab" is what often happens when you try to gain some extra distance. It is an unequal sudden application of power that causes the rod to bend, which shortens the "effective rod length", which causes the rod tip to dip down, which causes a dip in the fly line path (a dip = a concave rod tip path), which causes the tailing loop.
When you try for extra distance, the tendency is to also finish with the rod tip high so the fly line can shoot out the guides from a higher point to get max distance. This elevated position of the rod tip relative to the trailing fly line is what Dan Blanton is talking about.
What Dan Blanton is choosing to emphasize is that when a rod straightens after the stop, the "effective rod length" (the distance of the rod tip from the hand) lengthens. If you don't tip the rod tip down to compensate for this rod lengthening, the rod tip will be higher than the trailing fly line.
Although Dan Blanton says the rod tip travelling straight at the stop is the cause, we can see that the rod tip cannot travel straight because the rod lengthens at the stop as straightens from its curved shape before the stop. What he really means is that you need to tip the rod tip down to compensate for this straightening.
If we stop stop without the dip, the rod tip would go up relative to it's path just before. That upward curve would be a concave rod tip path. The concave rod tip path at the finish is what then leads to the tailing loop. So when he states that a concave rod tip path has nothing to do with a tailing loop, he chose to emphasize the cause of the concave rod tip path rather than the concave path, itself.
I prefer the Sexy Loop explanation because it describes all the factors that go inta a tailing loop.
One exception to Blanton's statement that a straight line finish cause a tailing loop is the Belgian Cast (the elliptical cast), in which the backcast is made low sidearm and the forward cast high with the rod upright. Because the two legs of the cast are made in different planes, a tailing loop with a "wind knot" will not occur. Even if the upper and lower legs of the loop formation cross vertically, they cannot catch on each other because they are separated horizontally in space. So there are exceptions.
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