It’s not some random force that happens to you.
The reason you get tense is because you are trying really hard to get it right.
It’s not because of your posture, or because of your technique is wrong, or because you’re stressed out. It would be great if there were such a simple fix.
It’s because you’re trying to do two things at once.
So, you have a choice: either let go of the need to get it right, or let go of the desire to play without tension.
This is not an easy choice. Of course, you want to get it right. And, you also don’t want tension.
This choice requires you to take a close look at what is really important to you. Not what is really important to your teacher. Your teacher does not live in your body and has not lived your life. So, there’s no way your teacher can understand exactly why you are messing yourself up.
If you don’t see how you are causing tension, you need to look closer.
(Check out my free course on how to pay closer attention to what happens while you practice.)