They have two special muscles. One is a sheet of muscle between the lungs and rest of the guts, beneath the lungs and attached to the shell, which squeezes the lungs to force gas out. The other is a sheet of muscle covering the rear opening of the shell (where the hindlimbs stick out). It's concave when relaxed, but flattens out when active, pulling the viscera back and expanding the body cavity. Since the guts don't change volume much on that short of a timescale (seconds), this forces lung expansion and inhalation.

Check the first two paragraphs and first figure of this paper: https://www.nature.com/articles/ncomms6211