Nokia is releasing Black update. But they are saying that Lumia 520 will not receive Glance Screen 2.0 feature due to hardware restriction.
Can anyone explain how Glance Screen would work and why Lumia 520 hardware is not compatible?
The Glance screen works without draining the battery too much due to the AMOLED screens. The Nokia Lumia 520/1/5 has a LCD screen, and thus is incompatible.
How it works is the Glance screen turns most of the pixels black. The AMOLED screens have the ability to turn off the pixels when they are black, so they aren't using battery by having the display turned on, unlike LCD screens.
I guess there will glance for lumia 520 soon as in the recent "Black" update there is a new extra under "extras + info" as Glance Pinboards App Agent 18.104.22.168. It looks like the hardware is made to match the glance screen and Microsoft is working on making the Glance app available.
As an anecdotal counter-point to all of the other answers here, I own a Lumia 520. It was purchased in the US from AT&T. I am not sure how much or even if the hardware differs much from Lumia 520's elsewhere.
When the device received the Black update, it received the new Glance features as well. I can see all of my app notifications on the Glance screen. Also, the double-tap to wake feature works on my 520, which I have heard others said is missing.
I want to mention, I do not know if this is a hardware or software difference, but it IS possible with some 520's.
I think that Nokia simply built cheaper screens into the Lumia 520 than e.g. into the Lumia 920 (which also has an LCD screen instead of a AMOLED screen,) to get the great price. Thus, it would simply drain your battery... On AMOLED displays the battery drain is the most minimal.