Unless you are shooting max distance, you should never let go of the line. You have dropped the line allowing the loose line to shoot. You have lost
line control.
Form an "O" ring with the thumb and forefinger of your line hand and shoot through this. It forms a another line guide but a guide that allows you direct control of the line. If the line is going to over shoot, you can close down the ring and the friction will slow the line down for a gentle landing. You can also control the shooting line so that it does not wrap around the reel as it shoots.
http://fishfliesandwater.com/2011/06...shooting-line/
I think of "line control" this way.
If you release the line from your line hand during the cast, you have given control of the line to the forces of nature.
Gravity and aerodynamics then control where the fly goes. What the O-ring technique allows you to do is to control both the line and the distance of the cast during the shoot.
Have you ever tried to land a fly accurately and the fly went too far or not far enough because you put too much or too little energy into the line shoot? Think of it this way. Could you park you car accurately by just using your accelerator and then having the car coast into the parking spot?
Of course not. You need a brake to slow the car down as it approaches the parking slot. For a fly cast, you can "brake" your cast with the O-ring.
Put a little extra power into the forward cast so that it would normally go too far. Shoot the line through the O-ring but then close your fingers around the line as the cast approaches the target. With practice, you can "feather" the cast so that the fly land right on target.
This is exactly what spin fishers do when they place their index finger against the top lip of the line spool to slow down the line or when casting reel fishers place their thumb against the casting reel spool. We can do the same thing by using the "O" ring shoot.