Suppose you need to cast a dry fly downstream. Of course you almost always fish up-stream with a dry fly; but perhaps you have just stepped into the river and see some water you want to cover below you. Obviously your dry fly will drag at once if cast downstream with a tight line. You can use a negative or positive curve, negative preferred in this spot, but there may be rocks, logs or fast current to each side that prevent using a curve cast.
In this situation what you need is a vertical negative curve cast with a wide loop or bow in the line on the forward cast. To make this fly casting technique, start with a low back cast made entirely with the middle of your rod and without accelerando movement. Carry the back cast too far back and don’t pause long enough for the line to straighten out behind. Make the forward cast also without accelerando action and with the middle of the rod -only.
Aim the cast very high in front and stop the rod too soon; do not use any thumb-pressure and shoot line before the forward cast has straightened out. Pull the rod tip back a bit at the end of the cast. This will cause your line to fall on the water with a good deal of loose line between fly and caster.
This cast also is useful for fishing the eddy of a pocket in a boulder-strewn river where the current flows up-stream, and therefore away from a dry fly fisherman who is working up-stream.