For me creep is just a name for a movement in fly casting, which (the movement) may be used for good, or which may support trouble for the next cast.
I understand this movement to be:
An early – mostly slow and mainly rotational – rod movement during unrolling reducing the available arc for the next cast.
If the arc left (after creeping) is just about the perfect size, then I don’t think it was a faulty cast – even if the creeping-movement was done unintentional. Of course the more details we know about (and understand), the better it is. But none of us knows it all, yet – I think!?
Back to the tailing loop (= a wave in the fly-leg of the loop crossing the rod-leg):
The most often cause for a tailing loop in my experience is:
– too uneven force application (somewhere within the casting stroke)
>> A partly concave tip path will happen (great illustration can be found here - by Aitor Coteron!)
There are other causes like:
– too narrow casting arc not matching the applied force (and thus resulting in too much rod bend for the size of the arc)
>> This one I don’t see often. Most students use an arc significant bigger, than it would have to be. Yes, even if they have creep (reducing the arc available on one of its sides) included.
Watching instructors trying to demonstrate tailing loops caused by creep (and trying to keep the cause to be creep only), I mostly saw them a) extremely creeping and b) stopping the rod pretty high. Thus shortening the arc on both sides! Also fair to say: Often the rod was lifted just prior the stop in addition and/or uneven force application was added as well.
Demonstrating (average sized) creep reducing the available arc for the next cast, and then stopping the rod in an average position (rod straight position ca. 10am) usually didn't lead to a tailing loop – as long as force application was a) smooth and b) not over dozed (and thus increasing rod bend unnecessary).
Besides all this not waiting for the line to unroll (bad timing), having slack in the system or violating the 180° rule (forward and back cast not inline) also may lead to increase the wave in the fly-leg. As a stand alone cause for (serious) tailing loops I hardly ever see this performed by students.
Yes, I know – this is contrary to a lot of stuff you can read on the www, find in books and DVDs. All this proves me to be right! ;)
Best is to not let any tailing issues creep in your casting though!
All my best
p.s.: If you are interested in a truly creepy (and long) thread written in straight tracking: Here you go.
p.p.s.: Always worth to rererethink such common parts of our fly casting. ;)
Just back from a great teaching weekend: