helsinkihelen, mmm I might not be the best gist-giver, but I'll try. This is more or less what I remember from the book/the theory of the approach (maybe someone who's reading it atm will come along and correct me
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As someone mentioned up-thread, the basic notion is that if you panic/stress during labour your body releases adrenaline, which is a fight or flight substance. This acts against the "nice, feel-good and relaxing" substances so your body tenses up and you feel pain. If you tense up the muscles that form your uterus and push the baby out tense up as well and that makes labour longer and more painful. If you relax, the muscles work as they're supposed to.
The idea is that the more relaxed you are, the calmer you are, the more you go with your body and let it do what it's supposed to do, the less discomfort you'll feel and the better you'll feel.
At least in my case, the fear of pain and the uncertainty of it all worried me a lot and I knew these would probably work against me. That's why I looked into HB.
If you focus on your breathing (breathing in in 4 and out in 8), on any visualisation that makes you feel calm and relaxed (this can be anything, from "a flower opening", "your baby descending", "a field with flowers", "water running downstream" as practitioners sometimes give as examples), or on some relaxing music in the background, etc, you might feel calmer and therefore feel less discomfort, more in control and better overall.
Visualisations didn't work for me, unfortunately. What worked for me was having the affirmations in the background and focusing on my breathing when a contraction came. Somehow it made time go quicker in my mind
That was long, sorry! Hope this helps, someone more knowledgeable might come along with better info 