Although it is true that some turtles have hinged plastons, in general the shell prevents normal expansion and contraction of the body wall.
Instead they utilize specially paired muscles. One set each for expiration and inspiration. The first set compresses the viscera against the lungs to exhale. The second set acts a bit like a diaphragm by contracting and pulling the viscera back up against the body wall ( thereby expanding the body cavity, without expanding the body wall ) for inhalation.
In addition this can be supplemented by rotating the shoulder girdle and pelvic bones backwards and forwards to help pump air. However, this technique is likely of secondary importance.