Hello, I can very much relate to this actually.
I am, by nature, also pretty tense. Also, It is very difficult to be concious about it all day long.
For me it is usually my (left) shoulder, which have been bothering me for a long time (by, unconciously lifting it up very slightly). My fysiotherapist couldn't fix this, I guess one has to solve this himself one day at a time.
I have 2 hernia's in my neck as well, which irritates the nerves of my left arm, this could be related but I'm not sure about it.
I have a desk job and it seems that was the culprit. My sitting position wasn't very ergonomical (I thought it was) and since a few month I adjusted my position and it is starting to get better. It still feels a bit unnatural though but that will change, I hope.
As for the original question: Yes, it does happen to me that my hand/arms feel 'free'. That awesome feeling of effortless playing, I wish I could hold on to it. Whenever it happens (which is sporadically, I try to pay close attention to what I'm doing, and how it feels. Sadly, I cannot trigger this by will on the next day. It usually happens when my mind is clear or very tired, could it be that I tense up when I'm 'overthinking' things?
Yesterday I was trying to play a scale slowly, with full relaxation, hands seperate. Should be easy, right? Well it wasn't and that's kind of frustrating. Any tips on relaxation in general? Or how to permanently hold that state of feather-light arms and fingers?
Best regards,
Carl