Why don’t Christians do this? I sometimes will rock back and forth in prayer, like at Adoration. I don’t do it to the same degree as the Jewish people I’ve seen, but maybe just a small rocking. I find it helpful for relaxing, concentrating on the Lord, and avoiding the distraction or sleepiness that sometimes comes with sitting for a long period of time very still in one position.
I saw a Jewish person doing it in a much larger rocking motion over King David’s tomb a couple years ago and it reminded me of when I was a child, some kids would rock to comfort or center themselves and there were even plastic TV watching chairs for kids so they could sit on the floor and rock a little while they watched. The kids across the street had them and one kid rocked all the time even when sitting in a regular chair. He took it a little too far but the point is that some rocking can be beneficial. So, why don’t Christians seem to do this?
(I have also read that Muslims are explicitly forbidden from doing it.)