The short answer is: "When you hold a cadence of 100 steps per minute, for at least 6-minutes".
POD START-STOP-PAUSE PARAMETERS
1) A “Session” starts when your cadence is 100 spm or more, for 6 minutes or more.
2) The Pod sees pauses and erases them. A “Pause” is a drop under 100 spm for less than 30 minutes. If a pause is longer than 30 minutes, the session will end.
3) A session is considered a “Run” when your cadence averages above 140 spm.
4) A session is considered a “Walk” is when your cadence averages above 100 spm and below 140 spm.
5) If a session is at least 60% “Running” (over 140 spm), the Pod will trim walks from the beginning and end of the session (i.e. warm-ups or cool-downs).
6) The Pod counts all walking and running within a session, as long as your cadence is above 100 spm.
7) The Pod counts every step towards the shoe life total (odometer feature) even if it’s not part of a Session.
NOTE: If your GPS device does not have auto-pause, or that feature is not turned on, you may notice a difference between the Pod duration and the GPS duration. The Pod reports net time while some GPS devices report gross time.