It's one of those things that may help, but unfortunately is presented as a "must". Same thing with relaxing your face, keeping your voice low, not gripping hard onto anything. Sometimes attendants get obsessive about mothers doing these things, as if anything else will keep the baby from being born.
Not necessarily. In my case, I had an extremely painful last six hours of birth, and the way I felt like dealing with it (that is, without thinking about it) was to scream and yell and scrunch up my face and squeeze the hand off my husband's arm. The midwife kept saying, "no, no, you must relax!" All of that stuff I was doing, though I couldn't seem to help it, was "wrong". Quite honestly it was demoralizing not to be able to do it the way she was telling me to (the "right" way,) and counterproductive to keep expending time and energy trying. I was trying to control my natural inclinations and make them something else. It was a VERY stressful birth and a difficult recovery, although there were no complications. I attribute that to the fact that in trying to follow her directions for the "right" way to labor, I was having to ignore what my body was telling me to do.
For my next I told the midwife (new) that I would do it MY way and I wanted her to stay OUT of my way. She was fine with that and didn't attempt to direct or guide my behavior in labor. After a perfectly enjoyable first stage, in transition the back labor started up again. But this time I felt free to do whatever I wanted, which included grimacing, yodeling at the top of my lungs, with my face stretched up to the sky, and biting the edge of the tub. It was my easiest, quickest birth; 2nd stage took 10 minutes. And this was because my way of "letting go" including making a big scene. (You can imagine how much I wished my first midwife were there so I could stick out my tongue at her and say, "so there!") I felt great after the birth.
My third was unassisted, and progressed in much the same way, only it was even more spontaneous and instinctive. I had a slow, mild, somewhat boring labor. Then in transition -- the last hour -- I again had back labor and felt compelled to make a lot of noise, thrash around a lot, swear, etc. Again, this was my way of "letting go", not holding back, as my first midwife had accused me of. The baby was born easily in two pushes -- which to me meant that my body was able to do its work efficiently and perfectly, that is, there was no evidence that it had been hindered by my actions, and of course I believe just the opposite -- that everything I did helped me release stress that the pain was producing, rather than creating it. I again felt great after the birth.