People are often unsure about the size of a particular amino acid. This makes it rather difficult to make guess when one sees density. Even for experts, it helps to concentrate the mind to see which ones could actually fit. It is necessary to have built at least a poly-alanine before this command works. Then the first time one ID's an atom, 2 knobs get activated so that one controls which amino acid type gets displayed and one controls the rotamer for that residue type. If the Poly-alanine does not point into the density, then the side chain will not fit into the density. Making further ID's, transfers the activity to the new residue.
At present, some of the commands are a bit dumb when it comes to accepting the results. Future releases will have enhanced functionality in Slider_combine and Slider_lego.