Here Fintan demonstrates an active release of the head from the chin rest while playing.
Paul Rolland discusses at length the notion of static tensions caused by immobility in any part of the body. One such tension is if we lock down the jaw on the chin rest.
YouTube offers a wonderful resource for observing how players such as David Oistrakh sometimes release their head from the chin rest. This YouTube clip is of Oistrakh playing Clair de Lune by Debussy, recorded in Paris in 1962:
Just as it's good to practise at times without the shoulder rest, also practise some passages with your chin off (especially passages without shifting).
Resources - Bibliography
- The Teaching of Action in String Playing by Paul Rolland p.32 - Principles of Movement in String Playing, Total Body Action