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.