Detuners are mechanical devices used to simplify the tuning of a stringed instrument during performance.
This allows musicians to quickly and accurately reach notes outside the normal range of their instruments.
For example, a relatively common arrangement on the bass guitar is to use a detuner on the lowest string to allow the bassist to switch between 'standard tuning' (E A D G) and 'drop D' (D A D G).
Bass guitarist and youtuber Charles Berthoud also makes extensive use of detuners, generally on a LeFay D-Tuner (which as the name hints was designed with a detuner on each string).
Adrian Legg is a popular guitarist making use of rapid tuning changes.