The upper thread tension can be adjusted continuously via the thread guide with pre-tensioning device and the thread tension device on the arm of the sewing machine.
Source:
Eberle, Hannelore; Gonser, Elke; Hornberger, Marianne; et al. (2022), Fachwissen Bekleidung, 12. Auflage, Haan-Gruiten: Verlag Europa-Lehrmittel, S. 211