I think you have to be careful with the concept of relaxed. I think it sends the wrong message sometimes. Actually most of the time. Agree with Mary.
In martial practice or situations you don't so much want to be relaxed but have the correct pressure and tensions in the right areas. Unfortunately most of us under stress revert to old ways we have learned to respond to pressure and tension exerted on us. We have a tendency to externalize our energies and push pull from our extremities vice our core and ground.
In practice we are instructed to relax these ares which IMO sends the wrong message and concept. It is not so much about relaxing, but transferring and shifting to areas where we can actually do something better than where we are.
This of course must be learned not from someone telling you to relax, but someone that actually has the ability to teach you how to develop your skills in this area.
IMO telling someone to relax is like instructing someone to drive a race car by telling them to "drive".