This is one of the things I hear most often when answering questions about class.
I think many people look at this the wrong way. I'm sure many other holistic practices get this all the time, finding balance, offering an outlet for the days frustrations, escapism etc.
Truthfully it's not as complicated as that.
How does adding another commitment to your life relieve stress?
How does following another's instructions alleviate tension?
Time constraints, mental taxation and physical exhaustion?
Not to mention the parameters of pseudo-foreign culture rituals...
Engaging in any physical activity and in my experience especially Karate only ADDS stress; so what I try to explain is by confronting ourselves daily we are serving to increase our resistance to stress.
By challenging the body you are encouraging it to grow.
By pushing the near limitless limits of your mind you are programming it to succeed.
By placing your self under extra stress, met with a determined attitude to overcome, you exceed the expectations you may have placed on your self.
Your body requires more energy caused by new demands so it does something amazing; it tells you to take in more fuel. Suddenly you have the energy adequate for your life, suddenly you have the mental capacity to meet the days requirements. You've evolved (in a sense).
So does Karate relieve stress, no, Karate is stress but intentional.
Karate increases your resistance to stress, and through it you find a way to overcome it.