Quakers were so-named (in scorn, at first) because they quaked and shook – in the early days, often literally – before the Lord.
I don’t usually quake so that you can see it, but there is a condition of inward or slight outward shaking which accompanies much spoken ministry in Meeting for Worship and in my experience also some moments in Pagan ritual and meditation (of many kinds).
I remember noticing my quaking particularly on one occasion: I gave ministry towards the end of a Meeting. I stood to do so, but finding myself somewhat weak for standing I leant my arms on the back of the chair in front of me. Sitting in that chair was a Friend well known to me, whose normal condition of life gives her a regular outward shaking. Feeling that Friend shake outwardly, and the weakness in my own legs, I was much more aware of the physicality of my quaking than I would perhaps normally be.
I can also remember quaking in private pagan rituals. (Usually the script takes away most if not all of the quaking in public rituals, though they are not necessarily less powerful for it.) Kneeling before my altar, naked, on a towel, and connecting with the forces of the Element Water by direct contact with a rather chilly sample of same – not all my shivers were outward.
I don’t take quaking to be the only or a required sign of true spiritual experience. I’ve had deeply meaningful experiences in which it was entirely lacking. I do think, though, that it is worth paying attention when it happens.