For some weird reason, the brain doesn't register the word... so telling someone to relax only makes them more tense.
"Softer" or "gently" seems to work better.
That's very interesting. I have to remember to try it at next class I teach - some other word... which will have to be in Swedish, though.
It probably needs to be positive instead of negative: "Relax" is like telling people not to use their muscles at all, which is impossible, whereas "softer" and "gently" indicate a certain way of using one's muscles, which is doable.