Relaxation is a catch 22 IMO. Agree with Mary on her post.
Relaxation can only come if you know how to get "unstuck", be free of the restrictions in a given form (technique). On how to get "unstuck", you need to know the technical details of the art which is the objective of your training. If you don't know how to get unstuck, the only other way is to muscle your way through.
But there is absolutely nothing wrong with muscling your way through in your beginning years because you will start to understand how your techniques evolve from a coarse technique (muscled) to a refined one (relaxed). If you can't discern the difference then there is an issue.
I think 100% of us at some point in time have been told by a sensei or partner while doing a technique "You must relax!". I didn't understand whenever someone said this and it still doesn't make sense to me now. It's only recent that I understood that relaxation only comes from knowing how to be free. It doesn't work the other way around.