This way of anal masturbation, which is said to come from the Orient, Villiot describes in his work »Les grands flagellés« as follows:

An ivory ball, on which a gut string is fixed, is inserted into the anus. The string then is tightened by another person and played with a violin bow. By that the ball is started vibrating which causes a stimulation of the anal nerves accompanied by a sensation of lust.

Sylvia Z. gave to me as a present the CD-ROM 'Bilderlexikon der Erotik', published by Wiener Institut für Sexualforschung, 1928 until 1931, new edition on CD-ROM, Digitale Bibliothek, Volume 19, Berlin, Germany 1999.

On this CD I found the text shown above.