The new invention proposes that filament yarns, in particular partially stretched yarns known as POY yarns, be subjected to stretch texturing via an air treatment nozzle. The air treatment nozzles are designed in miniaturized form, have a continuous yarn duct in which there open a plurality of transverse bores for the supply of high pressure air in the range over 14 bar, preferably within specific working windows between 20 and 50 bar. With the new invention, it was possible for the first time to process POY yarn via simultaneous stretch texturing using an air twister. The invention allows an individual thread as well as a parallel bundle of threads to be treated and permits for the first time the construction of a false twist stretch texturing bundle device with simultaneous air treatment of 500 to 1000 and more threads.