The first former guitar was made from a guitar and became something that is not a guitar, hence the name. They are no longer made out of guitars.
A former guitars have two strings that are share tension. A long “whammy” bar pivots at the top to change the tension of the strings inversely. One string tightens as the other string loosens. There is a movable bridge that acts as a fret and as a (reverse) bow. The strings are open when not being pressed or rubbed against the bridge. Pick ups amplify both ends of the strings so the inverse relationship between the string tension is combined with the inverse relationship between the string length above and below the bridge.
This configuration creates a tonal center and an equilibrium. The interval between the strings at rest is determined by string diameter. The range of timbre is very broad because of the tension control.