The Tuck Cast is an overpowered loop in the vertical plane, which results in the fly landing first and with slack. The height of the rod tip can be used to control the amount of slack.
It has a couple of prime uses in fly fishing; 1) it can be used to sink a nymph with slack line (and would be an alternative to the Collapsed Cast in this case) and 2) It can be used to place a fly down hard bringing attention to it.
From the manual [http://www.sexyloops.com/flycasting/tflyfirst.shtml]
…when fishing ‘pocket-water’. Boulder strewn rivers often contain so many varying currents that they present a bewildering puzzle. A simple solution to give ourselves that little bit more drag-free time is to land the fly before the line. So long as we can get fairly close to the pockets (and we can often get right up behind them) we can chuck the fly into position and attempt to keep the line off the water – of course the line will land eventually, but hopefully drag will have been delayed long enough for the deception to have been completed.
Let us also explore the second usage further… There are times when a trout is in a position where the only possible position to place the fly is behind it! Plopping down a beetle pattern in such cases can cause the trout to turn around and eat it. It is also a very useful method of bringing attention to flies such as grasshoppers in lakes. Chub are very susceptible to a “plop-landed” fly.
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