正 = Sei = Correct, right
座 = Za = Sit
正座 = Seiza = Sit in the right way.
In Budo the right way means: including Seichusen. Every posture needs to have an intact Seichusen, so that you can operate from a natural strong balanced position.
Seiza is fundamental position which you use to maintain Seichusen using natural tension. Provided that you do not have any deviation in your muscles and bones, a quality Seichusen should be possible. I specifically use the word quality, because every human (and animal and thing) naturally has a Seichusen. Whether this is 100% "in line", is another story.
Natural tension is in fact the same thing as natural relaxation, because both are caused by one another. If you have a bend or broken Seichusen or if there is anything only slightly "offline" when you sit in Seiza, you can simply solve that problem by shifting position. However, the issue is that this shifting is mostly associated with tensing certain muscles that do not solve the issue but merely change it. Eventually, it is a matter of training and becoming concious, like a baby can only squeeze hard or not at all in the beginning. Holding cutlery "correctly" lies somewhere in the middle. You need to hold it firm enough to use it, but not too lose in order not to drop it.