Is there such a thing as "active" relaxation?...

Playing lightly is perhaps a necessity for playing "well" - and I've tried various techniques to ease up on the pressure with limited degrees of success.  Playing buzz scales beforehand, or playing through pieces so lightly that they buzz is one method, but that has limited applicability when you have to apply a certain amount of pressure or your fingers sli___ide back together.  What needs to happen is for the pressure to release as soon as it isn't needed.  But if you're me, once you apply the pressure, it tends to stay on. This leads to a merciless build up of tension on progressing through a piece until moving at all is a major challenge. And then there's the additional problem of tensing up just because there is something challenging ahead - a fast shift, or a challenging stretch for instance - totally a recipe for not making it!
Why am I thinking about this now?  This semester I have a part in the orchestra that is a simple one line melody.  No problem, right?  Wrong!  It's very slow and very exposed, IOW a messed up note will be all too noticeable.  Compounded by a major shift 2 notes in.   Guess who tenses up and finds a straightforward shift difficult??
So I figured there is a need to be able to relax on demand. Easier said than done, of course and not something that seems to be taught as a separate skill. Once relaxed, the shift is easy, moving is easy, in fact the whole process of playing is much easier, so why can't I do it?  Probably because I don't actually know how to 'turn on' relaxation.  I've tried thinking about it beforehand but somehow it always gets lost in the playing of the music, thinking ahead etc, so I obviously need to slow it down. Not the whole thing, but precisely where I need to be thinking about relaxing.  So tonight I'm going to experiment with taking that piece and adding a beat before every shift with the goal of using that time to make my hand relax.   Maybe once I get that, I will be able to progress to relaxing without having 'thinking time'.  We shall see!