I see this almost daily: singers who tighten their neck muscles in a misguided attempt to "brace for action". It's completely understandable since most times when we call upon other muscles in our bodies to perform for us, we tend to tense them up. It makes logical sense and is a knee-jerk habit except when it comes to singing, it just doesn't work! In fact, just the opposite result of what we want happens - singing gets harder, not easier!
The moment we tense our shoulders or neck, the muscles surrounding our tiny and delicate vocal folds constrict and make the usable space inside our neck smaller. Smaller space means that our vocal folds have LESS room to vibrate and move against each other. This means less flexibility and less available vocal range. This particularly affects higher notes in the voice.
I often give students an activity to act as a "pattern interrupt" for their habitual neck tension. Sometimes it's trying to juggle scarves, sometime spinning a basketball, sometimes catching juggling balls or balancing a book on their head. I have lots of tricks.
This week, I had a young man with a fabulous voice come into the studio but the second he tried to sing above his break, everything tightened up and got strained. Being a basketball player, just the simple act of spinning the basketball was enough to get him to relax his neck and shoulder muscles and his high notes almost instantly became easier for him to sing.
Obviously, we cannot carry around basketballs to live performances but practicing tricks like this in the studio or at home can help create awareness and awareness is the beginning of creating lasting change.