which is outside the acceptable tolerance range caused for example by a defect in the bobbin 2 or a hindrance of any kind to the free movement of the yarn or a tension peak caused e g by the yarn being locked on the bobbin this operation prevents breakage of the yarn 1 and enables a substantially uniform quality to be maintained for the article produced by the machine t the method of the invention will now be described in relation to the use of the aforesaid device a yarn 1 is fed to the machine t to facilitate the initial movement of the yarn the braking member 15 does not initially act on it for example the discs 23a 23b are spaced apart by means of the sensor 5 the unit 6 detects whether the yarn is moving e g by sensing a continuous tension variation moment by moment using suitable algorithms this detection enables the unit 6 to sense whether the yarn is moving if however the unit does not sense instantaneous variations in the yarn tension it considers that the yarn is not moving and activates devices for halting the textile machine and or warning devices acoustic and or light emitting not shown to indicate the stoppage of the yarn and or its absence and breakage or its undue movement the device of the.