The strings play a vital role in badminton, afterall, they are the link between the shuttle and the badminton racket. Strings are probably the most important factor. When you first start out the strings are really not that important to you, and the beginners racquets usually come with factory fitted strings. As you are learning new skills this is probably just fine.
However, as you progress, the strings begin to play a more important role because they affect how you play and how the badminton rackets feels. Factory fitted strings have a low tension, mainly because the manufacturers do not want you breaking them, and they certainly do not want you breaking the racket. The low tension is to protect the racket frame. At low tension the frame is not under much stress and so is less likely to break.
Low tension strings can help you generate a bit more power because they produce a kind of slingshot effect. The shuttle will flex the strings more on impact, and then push back to release the shuttle. The problem with this is that there is less control, mainly because the strings will move on impact and so you do not have a solid hitting base.
As you progress you are going to get to the point where you need more control over your shots. When this happens you are going to think about re-stringing your badminton racket, and no doubt think about tension. It is very important to take things slowly at first. There is a misconception among badminton players to increase the tension because that’s what the professionals play with. There are a few things to consider before you start upping the string tension.
The first is the fact that at higher tension the strings are more likely to break because they are under more strain. The second is that the frame is more likely to break because it is also under more strain. The third point is that to generate power you need to be capable of making these strings flex, this means you need a sound technique and a fast swing speed. If you are a beginner or even an intermediate player your technique may not be good enough to get the most out of higher tensions. Another factor is that the sweet spot is smaller at high tension, so if you do not hit the shuttlecock in the middle of the racket, it will not travel very far, and you may even break the strings as well.
Only if your technique is good enough should you think about increasing the string tension in your badminton racquet. I would suggest increasing by 2lb at first. So if your original tension was 20lbs, which most factory fitted strings are, then go to 22lbs to begin with, and see how this affects how the racket feels for you. You you find the shuttle is more difficult to hit to the back from baseline to baseline due to the decreased sweet spot, but you should soon be able to adjust. What you should find is that you have a little more control over your shots, and power is nothing without control.
There is trade off here between power and control. There is no set rule at all becuase it all comes down to you. Some players can generate a lot of power at very high tensions because they have the fast swing speed, others will find it virtually impossible to get the same power because their swing speed is not fast enough. You have to experiment and find the right tension for your own game, and find the balance between power and control. Ignore what anybody else does, just concentrate on your own game and make sure you give yourself the best possible advantage to get the most out of your badminton racket.