A positive curve cast works best when your side cast is made with the rod at an angle of 45 degrees to one side. Let’s take the right hand side first. Make an ordinary right hand side cast pick-up, being sure you get a narrow loop on the back cast. Keep the back cast high while using this fly casting technique.
A sharp, short, left-hand pull just as the fly leaves the water helps on this point. Make the forward cast with a very narrow loop and with definitely more power than needed to straighten the line. Aim the cast about four feet above and to the right of your target.
Finish with a powerful thumb pressure that is applied longer than usual, longer than necessary just to straighten the line out. Another short, sharp, left-hand pull just at the time the thumb pressure is applied will help to give more in-curve to the end of the line.
Using this fly casting technique puts a lot of tip-action into the forward cast. When the rod gets to the finish of the cast, stop it abruptly and raise the rod tip slightly. The forward cast must be even more an accelerando movement than in the usual straight cast. This maneuver will cause the end of your leader and line to bend around to the left in an L-shaped curve.
Care should be taken to keep your wrist stiff during this cast, particularly at the end of the back stroke. In a 45 degree side cast, the hand is turned so the palm is at an angle of 45 degrees upwards. If you are using the wrist-action grip, you substitute a wrist motion for the final thumb pressure in the forward cast. Be careful to keep the rod angles correct. There is great danger of too much bending of the wrist spoiling the sharply defined rod angles needed for a good positive curve cast.