These string winding tools are essential for stringing up a Portuguese Guitarra. Called an alicate para torcer cordas, it is used to make loops in the ends of the steel strings. The Preston tuners require a very specific length of string, and the scale lengths of guitarras vary, so it is necessary to custom make strings so the loops on each end are just the correct distance apart. Now, string sets come with a loop on one end, but it is still necessary to put a loop on the other end.
These tools will work on any instrument with Preston (or watchkey) tuners, as well as other instruments that require a loop to be put in one end of the string.