The feature in Resmed is called Autostart. It does both, start when you start breathing into the mask and turns off the machine if you remove the mask. It can either be on or off, there is no separate controls for starting and stopping.
click to see full size
The Sleep in the highlighted area is when you turned on the machine "FOR THE DAY" and the Wake is the time you last turn off the machine "FOR THAT DAY".
The day ends typically at Noon.
In the above chart, there are 3 sessions.
So last night, at 21:34pm, I started with the machine for that day.
The first session lasted a whopping 17 mins.
The second session lasted 3h 21 mins
and the last session lasted 4h 15 mins
For a total of 7h 53mins for the "day"
The session colors at the bottom just change color back in forth to show a new session between blue and yellow.
You can also turn on or off sessions and then only see the sessions you have on in the charts on the left (not in the screenshot). Click on the green (or red) bar next to each session.