Auto Structure tool on Actions menu in the original interface.
The Auto Structures tool automatically assigns secondary structure to the protein based on the current shape or pose of the protein. Existing secondary structures are replaced.
The Auto Structures tool likely uses some version of the DSSP algorithm, which estimates hydrogen bonds.
Auto structures tool in secondary structure options in the selection interface.
In the original Foldit interface, the Auto Structures tool is available via the Actions menu. It assigns secondary structure to the entire protein.
In the selection interface, the Auto Structures tool is available when one or more segments is selected. Auto Structures is available via the secondary structure tool icon or the lowercase "l" keyboard shortcut. Only the selected segments have auto structures applied. The secondary structure of any segments not selected is not changed.