String tension is crucial to performing tricks, especially when you're learning new tricks. Tricks like iron whip can be particularly hard to master if your string twists when you try to do the whip.
If you notice your string wants to twist up when it goes slack, it's either gotten too tight or too loose. Imagine throwing sleepers one after another, or practicing Trapeze. Each time you throw, you're twisting the string a little more in the same direction. For a right-hander, the winds in the string will tend to tighten up, while for a lefty, it'll get more loose. Eventually it'll get to the point where you'll feel like you need to take the loop off your finger and untwist the darn thing.
Getting the right string tension
There are a few tricks you can do (UFO/Flying Saucer, Lariat, Sidewinder, are three examples) that will either tighten or loosen your string depending on the direction you throw them. I use the UFO/Flying Saucer, so that's the trick I'll use as an example.
First of all, you need to know whether your string is too tight or too loose. You can quickly gauge this by holding the unwound yo-yo so that the string sits horizontally as in trapeze. Bring your hands together a bit to give it some slack, and pay attention to the direction in which it tends to turn as it tangles up on itself. Looking down on the string, if it starts twisting on itself clockwise, it's too loose. Counterclockwise, it's too tight.
If you throw a UFO across my body (from right to left), you are loosening the string. If you are right-handed this is what you will be doing the majority of the time. But occasionally (or if you are left-handed) the string will be too loose, and to tighten it, you throw the UFO from left to right.
|Slack twists...||String is...||Throw UFO...|
|...clockwise||...too loose||...away from your body (left to right) to tighten|
|...counter-clockwise||...too tight||...across your body (right to left) to loosen|
When you check the tension of your string you should also inspect it for wear. If the string starts to fray or become dirty and greasy you should change it. A good yo-yoer always changes his/her string long before it snaps.