Fringe lockers can be used to diagnose problems with a set up. The output is sent to a scope to find out the frequency of the problem.
They can also be used to correct very small instabilities with set ups. Depending on the movement capacity of the piezo element or piezo element stack, the fringe locker can compensate for movements from a few fringes up to 10's of fringes. The fringe locker used for corrections is most widely used for long exposures. The types of movement they can compensate for depends on the frequency response.
They work on the principal that a fringe from the setup is expanded with a lens so that one fringe is in between two photodetectors. If the fringe starts to move a difference in voltage between the two photodetectors is noticed. This signal is sent to a piezo element under a mirror in one of the beam paths in order to change it's length and hold the fringe still.
A commercial example is the Stabilock.
A home made circuit from Joe looks like this: