Jack almost certainly didn't know, but tracing Gallifeyian into his skin was a move the Master associated with the Doctor trying to keep him still and calm, and focused on the Doctor, particularly back in the bad old days. Or maybe Jack did know, and best for the Master not to dwell on why.
At any rate, the Master was practically conditioned to stay still under Jack let him go, his breath slowing, his mind dropping into even more receptive state, although automatically trying to make sense of the patterns.