I mean is there any pieces that should be played with tension and better sound ffect.
Eg,relaxation for fast pieces and tension for slower pieces , forte ,etc
The secret to a really good technique seems to be to:
1) activate what you need and keep it activated;
2) "relax" what you don't need and keep it "relaxed" (= not more tonus in the muscles than is absolutely required).
P.S.: Correct coordination (mostly a neurological problem) and true independence of different body parts is crucial if you want to solve this.
For example: most people are not able to lift their fingers one by one without experiencing tension in the other fingers, the hand, the wrist, and ultimately the upper side of the underarm. This is a NEUROLOGICAL problem that *can* be solved if you have enough patience, but it is not proof that lifting the fingers in itself and swinging them gently into the key is bad practice for all as some piano methodists proclaim.
Another example: when one plays a chord, one should check whether fingers that do NOT participate have freedom of movement. Can you "wiggle" or "waggle" those fingers? This is also a neurological problem. Instead of running away from the problem by inventing misguided theories about "tension-relaxation" of fingers that should stay active, one should simply solve the underlying neurological problem of true independence through smart practice, preferably under qualified guidance.