I wear an HRM-TRI during martial arts practice. Due to safety considerations I can't wear the wristwatch, but the chest strap is fine. So I start an activity, take off the watch, and continue with my workout. This works great for tracking heart rate and syncing up with the activity afterwards, but the data from the HRM-TRI does not count towards intensity minutes.
For example, today my workout had a Training Effect of 3.5 Aerobic, and 3.2 Anaerobic, and I exceeded what I thought was my max HR, but yet I have zero intensity minutes for the day.