predetermined basic tension in the curved part 18 in the detection position the tensiometer t measures the yarn tension downstream of the yarn brake b then with the help of the measurement result or the measurement results of the representative number of insertion cycles respectively the withdrawal brake 2 and the yarn brake b and in some cases the detector d can be set so that the optimal curve 15 of fig5 results those adjustments can be carried out manually or in a closed regulation loop by means of automatic regulating devices which are not shown in detail e g guided by the control device c and or the control device cu in this fashion the curve 15 is established as soon as this has been done the yarn processing system may start the normal operation in the case of a strong yarn which does not have a tendency to break despite the engagement of the tensiometer the tensiometer t is kept in the detection position this allows to monitor the normal operation with the help of measurement results and to carry out in some cases further adjustments or optimisations in the case of a weak yarn however the tensiometer t then is switched over to the passive position in order not to influence the yarn further on if necessary in case of the occurrence of irregularities or when the quota of yarn breakages increases or even regularly only for u201c checking purposes u201d e g with each 100000 th insertion the tensiometer t then may be switched over in to the or into one detection position respectively in order to then carry out re adjustments the shown tensiometer operates according to the principle of yarn deflection by the element.