Shuckling refers to the ritualistic rocking motion made by Jews during prayer.
(Note that this isn’t a situation where I found a word and looked it up. Rather, I observed something and assumed it had a name, so I went looking for it.)
This page has a lot of explanations for it, such as:
- being so moved by prayer that your entire body becomes involved
- the need for exercise during long periods of prayer
- when multiple people prayed from a single Torah, they would constantly stand up and bend down, leading over time to the ritualistic swaying
I gather it’s one of those things just passed on by custom, and everyone might have a different reason for it. Over time, it likely has just become a communal norm – you do it because everyone else does it, and it becomes a rite of passage or membership.