Relaxation:
Generally, I try to feel that my torso is relaxed along with the upper arm and the forearm. The wrist should be completely flexible, being able to be readily rotated by the relaxed forarm, to flex verically and laterally, thereby angling and directing the hand. The fingers, though, cannot be relaxed, as they would become "cottony" and not very responsive, fast or accurate; instead, they need to be kept taught or firm in order to articulate.
There are some more specific things I do to encourage relaxation. First, I try to keep my shoulders naturally and comfortably level. If they start to rise upward, an inner monitor senses that and lowers them automatically. Secondly, it's essentially too that the elbows be in free float so that they can travel away from the body as needed to do their part (along with the torso and upper arm) to direct the forearm along the keyboard. If the elbows are tucked in close to the body, then that directing activity cannot happen very easily or effectively.
Finally, during intensive practice, the playing apparatus can sometimes tighten up. If I sense that happening, I stand up and swing my relaxed arms in parallel at my sides front to back several times. Then I swing them directly in front of me such that they cross one another. Finally, I let my arms fall to my sides where I do a rapid shaking, rotating motion of the arms, as if very rapidly jiggling two door knobs with my hands (or imagine two loose ropes dangling in the wind). This brief routine immediately dissipates tension, tightness and fatigue. I find that the better the relaxation, as I've described it above, the richer the tone I produce.