If the yarn hits the line because the leader seems to whip, pull or fall downward as it unravels, it's a tailing loop as described by others.
There are several ways we can create one of theses little buggers, and to really correct root cause it requires some face to face with an experienced friend or instructor - but there are also some generalisms that are worth mentioning even without having seen it happening... (I'll refrain in this case from going technical stuff about path of accleration, tip deflection, application of power, etc)
Make sure the rod tip "ducks" under the line coming over the top at the end of your forward cast.
In other words, as you come to the stop, make sure to turn the wrist so that the rod tip tucks just below and thus out of the way of the line flying overhead toward the target.
Not too far downward or it will open up your loop. If you practice this you'll find the right amount, just a bit will do it.
Try that first and we'll go from there.
IFFF Certified THCI @ 2005
Capeflyfisher Guide Service
Island Hopper, Guitarist, Incurable Dreamer
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