For cutting dentin by Gates Glidden drills, it should be used at:

It is ideally used to cut dentin as it is withdrawn from the canal.  It cuts on the outstroke, and it is used to open canal orifice.  It is used in 750-1000 rpm. Long shaft is designed so as to break at the neck, the narrowest diameter that lies adjacent to the handpiece.  If the drill binds during use, it will fracture at the neck of the shaft.