Given as part of an answer to a question put by Ian Stephenson on approaches to learning the Harvey:

The Zimmermann Sonata is a different matter, as the piece is so heavily marked with performance indications of different kinds. The performer must make choices about how to interpret the composer's indications and the issue of timing is subjective too eg. how should the fragments be presented - fused together completely, clearly broken up, basically joined but with selective breaks of varying lengths etc. etc. Of course, the Zimmermann does not present the soloist in the same way as Harvey does. Harvey is using the player very much in the "romantic" sense of the soloist: emphasis on lyricism, temporal freedom, traditional virtuosity (such as fast scales and runs) and exploitation of the traditional expressive qualities of the cello. Zimmermann is more concerned with the use of the instrument and instrumentalist as a kind of laboratory of timbre and texture where the articulation of serial techniques is uppermost in the composer's mind ("skeletal treatment").

NB I'd be interested to know about the technical demands of the piece, particularly directions for the use of the bow and the alternating bow/fast pizzicato passages.

GP The main technical demands are fast changes from bow to pizzicato, many quick leaps around the finger board. The challenge here is to maintain the flow of the music despite the intrinsically fragmented nature of the material.

The Cello Sonata is from Zimmermann's finest period, the last ten years of his life (1960's), when he was very preoccupied with experimentation on all levels.