In short, your body affects the signal. And through MIMO there are multiple simultaneous signals being sent between different antennas on wifi-devices. With jus

In short, your body affects the signal. And through MIMO there are multiple simultaneous signals being sent between different antennas on wifi-devices. With just one signal, wifi sensing doesn't tell you much. You sort-off have to look at signal loss and guess if it is lower because the signal is passing through you. With multiple signals, not only do you get multiple data points on signal loss, you also get a phase difference between the signals (i.e. a signal that passed through you is delayed a bit compared to signal that didn't pass through you). By integrating that knowledge from various signals (or rather 'channels') and probably doing a whole lot of other smart stuff, potentially some training / fingerprinting (i.e. if they are at position X, then the signals do Y, for many different positions), you can get location data.