Arrakis it's a set expression literally "don't jerk yourself, stop twitching" used as "don't worry". Used not when someone shakes in your view ;o)), no epilipsia, but when you suppose someone worries about something very much and I guess is torn mentally between several options what to do.
Another circumstance is when one plans to do something not thought through well (it seems to you), jump at doing something, so you stop him, like, don't jump at solutions, don't take rush? decisions. Well I guess this is then the same as idea 1.
Never thought of it when we use it.
I would hear it often :o) Like, I would say, well, if I can't make it for that train I will thyen run borrow money er? soemwhere and return that ticket and buy another one for two days' later and, if I also simultaneously ... etc!" and someone approx. Mum :o) would say stop it slow down stay at home simply that's too complex planning nothing that is done quickly is done well. :o) So this is "slow down" as well.
Well then this is an all-circumstances embracing list at max.