Here is an explantion from the Logitech forums on this feature:
"Please find below the details of Harmony 800 series back light function.

The motion sensor is a ball bearing that is surrounded by 4 metal posts.

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The firmware only recognizes the bottom posts. When the ball hits the bottom 2 posts, the back light comes on. The bottom 2 posts point directly toward the IR learning port (or the opposite side of the USB port), so when the remote is lifted towards a persons face, the back light glows. You will notice that if you hold the remote flat on a table, and then point it towards the floor, the back light will not turn on.

The motion senor found in the 500 series is a spring coil encapsulated in a small metal tube. This piece is very sensitive, and works on vibration. The difference between the tilt sensor and motion sensor is that the motion sensor does not turn the back light on. Once the spring vibrates and touches the tube, the firmware takes note of vibration and starts a 2-hour count down. If no additional vibration has occurred after the 2-hour period, the remote will go into deep sleep mode. Deep sleep mode will turn off the LCD and draw much less battery current than normal usage. If vibration is sensed while the remote is in deep sleep mode, the back light and LCD will turn on again, and the remote will boot to the last state it was in (ie. the last activity)."