Smart Scroll is a fair bit gimmicky. I can get it to work with just my eyes only holding the phone at eye level and in good light, but who holds their phone like that? It does also track the nodding of my head - I look like I'm agreeing a lot on a video call!
Toggle it on and off from notification drawer>extra toggles (top right corner) and the Smart Scroll toggle.
Samsung Galaxy Note 3 SM-N9005
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