Spin Control
It generally only takes an outing or two to gain some directional control and start to have a lot of fun with your sling. As you continue to refine your ability you may start to notice curve in your projectile travel, particularly with a tennis ball. Excessive curve will limit your range and accuracy and you will want to learn how to control it.
To understand what is going on it is important to understand that slinging always imparts a spin on the projectile. This spin occurs as the projectile leaves the pouch upon release and the pouch and a portion of the release cord slide across the projectile. For a right-handed slinger, spin will be clockwise to the slinger and opposite for a left-handed slinger. This spin or rifling can be used to your advantage and contribute to projectile accuracy and distance as the axis of projectile spin is centered on the path of travel. Conversely, when projectile spin is not in line with the path of travel it will result in the projectile curve.
Fortunately, the principle of spin control is pretty straightforward:
If the projectile axis of spin is in line with the path of travel then it will rifle straight. Think of a straight stick through the middle or axis of a spinning tennis ball and imagine that stick in line with the path of travel. It rifles straight ahead. But if that stick (projectile axis) is pointed up or down slightly at release then the ball will curve. Because the pouch orientation determines projectile axis orientation you will want to consider how the pouch is oriented at the release point. If the pouch orientation puts the projectile axis in line with the path of travel at the release point, then you will have a rifling shot (Fig A).
Figure A
However, if the pouch is oriented or tilted away from the path of travel (upward Fig. B, or downward Fig. C) at the release then the projectile axis will have the same orientation and its spin will be out of line with the path of travel causing it to curve off course.
Figure B
Figure C
If you find that you are fighting curve to the right or left then try this: Rotate hand orientation slightly before release, either more palm down (pronate) or more palm up (supinate) so that at the release point the pouch orientation has changed. The slight change in hand orientation will change pouch orientation and the spin orientation of the projectile. Practice this slight change until you notice the curve being eliminated. For most slingers the curve culprit is a pouch opening facing slightly up at release and they will need to pronate (palm down) more so that the pouch lines up straight ahead at the release.
Keep in mind that no matter how good of a rifling shot you have if you throw angular or oddly shaped rocks you will get some moderate to severe curve.