An improved system for fabricating alternating S/Z cabled yarn, for alternating S/Z twisted yarn and for (whether or not alternately) applying a torsion to a yarn, in which critical air flow conditions are obtained at least at the outlet of the torsion chamber of the devices. A method for fabricating alternating S/Z cabled yarn, for alternating S/Z twisted yarn and for (whether or not alternately) applying a torsion to a yarn under critical air flow conditions at least at the outlet of the torsion chamber used in the method, and a product according to these methods.