The Idealize SS (secondary structure) tool creates a helix or sheet with "ideal" angles. Idealize SS uses the secondary structure as currently defined, and does not take into account the amino acids involved or the position of the sidechains.
In the original interface, the Idealize SS tool is available in Pull mode, via the right-click (control left-click) context menu. It idealizes the secondary structure adjacent to the clicked-on segment.
In the selection interface, the Idealize SS tool is available when one or segments are selected. Idealize SS can be accessed by clicking on its icon or using "5" keyboard shortcut. The tool is applied only to the segments selected. If you double-click on a sheet or helix segment, the entire sheet or helix is selected, allowing the tool to process the entire secondary structure.
The Idealize SS tool has no effect on loop segments.