Strings are one of the first things that advanced players change when they purchase a new racket, so much so that in Asia most rackets are in fact sold a frame only and then strung to the players requirement.
New rackets usually come with factory strings, which are normally strung at a tension of around 20-22 lbs. This tension is ideal for the beginner or improving player, the reason why this is, is because a relatively low string tension gives a large “sweet spot” on the face of the racket where the player will be able to achieve optimum power and precision over a larger area of the racket head. Lower tensioned rackets are ideal for players who lack in power in their arm and therefore benefit from increased power from the string bed with in most cases a relatively slow and long arm action. Low string tensions however aren't ideal for the advanced player who have a fast “punchy” action and that can provide power from the arm and improved technique.
A higher string tension of around 26lbs+ makes the shuttle spend less time on the string bed and reduces the sweet spot, therefore requires more skill and consistency to achieve accurate shots and gives the player a lot more control over the shuttle, however to achieve power from high tensions arm power is required. This is one of the most common reasons for injuries in badminton, players believe high tension gives more power, they then suffer from arm injuries trying to put power into the shuttle.
After a while, 6 months or so, you will find that your strings will become lose, this is due to the strings stretching over time loosing around 1-2lbs in tension. It is normally recommended that you have your rackets restrung every season.
Badminton Alpha currently stock the Kevlar 10000VS with its 35lbs guaranteed string tension, however tested up to 40lbs in tension. This is great for players who can provide power with a consistent and accurate stroke.