The yarn tension unit is the part of the knitting machine that feeds the yarn to the knitting carriage.
Its main job is to keep the yarn tension constant, since the tension directly influences the stitch gauge.
If the yarn tension is too low, stitches may even fall of the needles resulting in ladders or runs. Extremly high yarn tension exerts high stress on the needles, possibly bending and breaking them. These extreme cases may occur if the machine runs out of yarn, or the yarn gets stuck.
While industrial machines and automatic carriages have mechanisms to stop the machine when the yarn tension is out of limits, home knitting machines have a kind of antenna with a very long flexible wire providing the yarn.
This antenna is a dynamic yarn-storage, that makes sure the yarn-tension does not increase abruptly when the yarn gets stuck. It provides the knitting machine operator with enough time to stop moving the carriage, without destroying the machine.