The straight line is one of those things in fly casting that you have to learn very well in order to forget very quickly when fishing. There are three very distinctive kinds of straight line cast:
Useless: it produces drag immediately
Necessary: in active presentations that require lively imitations
Essential: as the starting point in many presentation casts and very long casts (you know, for the crazy guys with their WF5s)
The opposite of a straight line is a line away from your target, or line which is slack. It is important to understand that even a slack line cast must start with a straight line or else it will end up being a shapeless and rather useless heap of line. Also, don't forget that in order to be accurate with a slack line cast, you first have to be very accurate with an overhead straight line cast. And that's exactly what you are going to practice with this exercise.
Two rod cases
Objective and instructions
Place your rod cases ten metres away from you totally rolled up and 50 cms apart. Your goal is to lay the line in that corridor without touching either of the cases. The butt section of our leader must always land at the far end of the corridor.
Once you're hitting the target an average of four times every five attempts, you can start to unroll the cases gradually. This way the exercise becomes more challenging.
Try to keep the whole system in a perfect straight line. With a very closed stance our body, arm, wrist and thumb must follow a straight trajectory in line with the cases. This position will give rise to a vertical loop, which is the most useful kind for this exercise. See what happens if your thumb tilts a little to one side after the stop.
Things get tough
Step back away from the target (11, 12 metres...)
Put the cases closer
Kneel on ground.