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It is not possible to know what sensor they used to develop this app. But HRM will be best option among your given list. Because it directly get data from human body. If the body is not connected with the gear, it can't measure the heart rate data. HRM is included in Human Activity Monitor of Tizen API. There is also a api called 'Wrist Up' detection to get the wript up event. 

You may follow https://developer.tizen.org/development/guides/web-application/sensors/human-activity-monitor

Yes, Andre Reus is right. The best way is to use HRM sensor. Wrist Up is not appriopriate here, it says you when user watch on the display, though.