that the feeding fingers are controlled synchronically with the control of the device according to this invention as discussed hereinafter in normal conditions when a yarn as fed to the needles through the yarn feeding finger 3 is regularly knitted the element 22 is accommodated in one of the notches 24 of the cam 23 and the arm 13 is at a rest or inoperative position such that the roller 29 is spaced from the cam 31 and is unaffected by the boss 32 while the cam 31 continues to rotate at the same rpm as the needle cylinders 5 6 the movably supported valve 11 of the respective feed is closed and no air jet is blown from the nozzles 4 and 7 when one yarn must be replaced by another there occurs a movement of the yarn feeding finger and gripping cutting device as described respectively in the cited u s pat nos 3 605 444 and 4 099 392 the control impulse which actuates the new yarn feeding finger also produces the synchronized operation of the blowing device according to the invention in fact the energization of the electromagnet 51 determines as mentioned already the engagement of the entraining tooth 42 with the entraining notch 43 and consequently the rotation of the gear wheels 37 and 35 in the present instance having assumed that the cams 52 52 39 and 53 53 39 control respectively the yarn feeding fingers and gripping cutting device.