Anyone know how my machine (resmed s8 elite) calculates a hypopnea? I understand that an apnea would be ten seconds without  a breath. But if hypopnea means breathing 50 percent more shallowly, how does it know?

Thanks!

Basically, there is a flow meter inside that measures the amount of airflow of the machine.  If the flow changes because you stopped or reduced your breathing, it can tell.

From the ResMed Clinicians Manual:

APNEAS AND HYPOPNEAS
An apnea is defined as a greater than 75% decrease in ventilation. The device algorithm scores an apnea if the 2-second moving average ventilation drops below 25% of the recent time average (time constant 100 seconds) for at least 10 consecutive seconds. A hypopnea is defined as a 50 to 75% drop in ventilation. A hypopnea is scored if the 8-second moving average ventilation drops below 50%, but not below 25%, of the recent average for 10 consecutive seconds.
The AHI, AI, and HI reported in the Efficacy Data submenu are calculated values per usage hour per session, or median values for a selected time interval (last week, last month, last six months, or last year).
Note: When EPR is enabled, detection of apneas and hypopneas may become less reliable at high levels of leak (> 0.4 L/s — 24 L/min).