Which is wound around it forming coils, thereby countering the centrifugal force that is the main cause of balloon formation.
This problem, typical of ring spinning machines since it derives from their very working principle, is very evident in the case of woollen and semi-worsted ring spinning frames, due to the coarseness of the yarn and the large size of the spool and ring. It is known that the factors negatively affecting yarn tension are essentially related to the size of the balloon, to the square of the rotation speed and to the various sources of friction: yarn-air, traveller-air, traveller-ring.
The coils wound around the spinfinger form a sort of reserve that supplies material when yarn tension is higher (winding of the kernel and on the smaller diameters of the frustrum) and gathers it in again when the tension drops. This allows it to cope with periodic changes in winding tension, eliminating the balloon totally (this is the solution adopted in the case of coarse yarns) or partially (the solution preferred for medium-fine and fine yarns).

The main difference between the two types is the path followed by the yarn, which, having formed a few coils around the spinfinger, runs around the tube to reach the spool formation level (this is in the case of total elimination of the balloon), while in the case of partial balloon elimination it is detached from the tube, forming a smaller balloon.
Clearly, therefore, to avoid the risk of fibres getting caught and breaking, the tube has to be well finished. The adoption of the spinfinger has allowed an increase in the tube length, with consequent benefits in terms of productivity.
Furthermore, the spinfinger allows the twists to be more evenly distributed, reaching as far as the delivery cylinders at the end of the draft range: sometimes, friction with the thread guide causes local elimination of twists and thus a considerable weakening of the yarn and increased risk of yarn breaks.
It is worth noting that modern, electronically-controlled machines use continuous spindle-speed variation during the formation of the bobbin (kernel, build-up and end), as well as during the coil deposition and winding/binding, in order to keep the yarn tension constant.