When viewed from above the fly line in the forward cast must follow a straight line that is the continuation of the straight line draw in the back cast; that is, the fly line in the forward cast must be 180º from the fly line in the back cast.
If those trajectories in the horizontal plane don't draw a straight line we will have problems. When the difference of the respective trajectories is big we'll have big problems, like those shown here.
Excessive consumption of liquid has problems also, as shown by the beer belly. :-)
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