Oh, if only this were true...
When it comes to tension and letting go of tension my experience (as an Alexander technique teacher) is that the body does do what we tell it to. The problem is knowing exactly what to tell it to do!
Telling it to "relax" isn't exact enough to be very useful. Unless with practice "relax" has become short hand for a list of useful things that lead to "relaxing". And even then relaxing is of limited usefulness IMO.