Doppler shift of the signal against the expanding and contracting thorax. Through static media which can be largely calculated out. Movement of 2 to 7mm is significant enough to detect with a precision UWB STTW radar system, and even easier when it's periodic.

That should be 1/3 - 1/6 Hz signal, ~3 to 6 seconds/breath.

Most of the literature I've read regarding STW and STTW radar monitors the doppler shift on the return signal looking for a periodic ~3 to 6Hz signal corresponding to the typical respiration rate of humans. When found, the source area is flagged for more precise imaging. The motion of breathing is usually subtle enough to be skipped by larger scale motion detection algorithms, and is labeled differently accordingly.