Shake Sidechains is a command used in GUI recipes in Foldit. It appears as "Shake Sidechains" in the list of commands, then "Shake" when it's selected.
Shake Sidechains uses one ingredient, iterations. The possible values for iterations are "until stopped", or an integer from 1 to 25.
The conventional wisdom in Foldit is that shake completes all its work in the first iteration. It's common to specify one iteration, although some recipes use two iterations to be on the safe side.
The "until stopped" setting doesn't have an equivalent in a Lua recipe. Lua recipes require the number of iterations to be specified in advance. (The user can interrupt a command running in a Lua recipe, however.)