I studied wind knots a few yrs back mostly so I could control them to be able to pass a test. I came up with an understanding/description/conclusion, let me know what you think.
First off I have a problem with most books which talk about what to do with your thumb, arms, shoulders, elbows or how you hold your mouth open but in the end I've found all that to be much less important than dialing into what you are actually doing to your line.
Of course the former may indeed effect what you are doing to your line (except maybe your mouth position) but you could also stand on your head and cast without a single tailing loop with a rod taped to your feet if you move the line correctly, or do everything wrong with every part of the body standing upright and still have no tailing provided you move the line properly.
You could have perfect thumb, elbow, arm and shoulder posture but still get a tailing loop if the direction of the line at the final stop is higher than the direction the line was pulled during the casting stroke.
The cause of a tailing loop is simply this - the line is pulled along one vector during the casting stroke but the direction of the final release vector is higher at the stop.
You can pull the line downward during the casting stroke and stop perfectly straight and you'll get a tail, or you can pull the line perfectly straight during the casting stroke and pop the line upward in the end and get a tail, or worse yet - you could do both and will definitely get a tail.
Conversely, as long as the final release is lower than the line's vector during the casting stroke you will never get a tail even if you are standing on your head and casting with your legs.
BTW - the gap between your casting stroke vector and final stop is roughly equivalent to the size of your loop (if you like tight loops).
A visual aid...
Imagine a train heading full steam into a big dip in the track. The train's momentum is pulled downward into the dip, then rises back up the other side.
The train's momentum is going down hard but is able to climb upward by pushing very hard against the steel rails and the hard ground on the far side of the slope. The passengers stomachs will attest to that.
Now remove the tracks and pull the train through thin air quickly through an imaginary dip (like a fly line). Once the train is pulled down into the imaginary dip, no matter how hard "the little engine thinks he can" pull things up, the established downward momentum in conflict with the late upward shift will eventually form a tailing loop out of the remaining cars and caboose.
Whether just a sag from depleted energy or a full-scale whiplash, that conflict has to resolve itself somehow and that's exactly what the fly line is doing when it tails out.
Likewise the line could be pulled perfectly straight during the stroke but the final stop may be aimed too high creating the same conflict and a tail will result. So both are required to prevent a tail.
Causes of the downward pull during the stroke:
Well As Dpolipo said if you haul off-time or if you haul very hard it will deflect the rod too much and pull down the line in the middle of the stroke. This is especially true with soft rods (but stiff rods have their own problems too).
Or you just might aim too high with your final snap while trying to do a moonshot, in which case the direction that the line was originally going is not parallel to the final release direction and you will be rewarded by a vicious tailing loop.
Make sure that you pull the line thru a vector during the casting stroke that is parallel to and higher than the direction of the final snap at the end of your cast.
Lefty says the same thing when he says to stop the rod tip beneath the line.
BTW2 - For distance, lengthening the stroke length and increasing the rate of acceleration over that long stroke will result in much more distance than muscling it ever will.
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