A 2018 Foldit blog post New Foldit tool: Pick Sidechains introduced the tool.
In the selection interface, selecting one or more segments displays the Pick Sidechains icon and also makes the tool available via the lowercase "a" keyboard shortcut.
While the tool can process multiple segments at once in the selection interface, in the original interface, it handles a single sidechain at a time.
In either interface, the tool displays a moveable, resizable window showing a list of the segment or segments selected.
Clicking on a segment shows the rotamers for that segment. The blog post describes this:
Every rotamer in the list is labeled with a shorthand name, using the letters ‘m’, ‘p’, and ’t’ to describe the rotation of each bond in the side chain (for “minus”: -60º, “plus": 60º, or “trans": 180º).
The panel also includes a gauge for each rotamer’s prevalence. Certain rotamers are more common than others, and these are typically preferred for protein folding. A full gauge indicates a common rotamer, while a rare rotamer will have an empty gauge. More common rotamers will usually score better, but sometimes a rare rotamer can make an excellent hydrogen bond or fill a void to gain more points!
In other words, rotatmers have names like "mm", "tp", and "tm". There can be several rotamers with the same name, however, each with a slightly different position. Names like "tttt" or "mtmt" are also possible for longer sidechains, which have more bonds.
The tool also shows all possible rotamers for the selected sidechain(s), as a "cloud" or "superposition" with multiple positions superimposed.
The Pick Sidechains window doesn't have a close button, instead the tool must be cancelled with the escape key or the space bar.
Clicking on a segment in the upper part of the Pick Sidechains window show the possible rotamers for that segment. Dragging on the dotted bar at the bottom of the window expands the window to show more sidechains, and the list is also scrollable.
Clicking a particular rotamer moves the sidechain into that position. Sidechains can be moved this was as often as desired.
Closing the Pick Sidechains window leaves all sidechains in their most recent position. Again, the windows can be closed using the escape key or the space bar.