1.) If you've watched Angel Beats (the main story), this is a must watch. It explains most of the unanswered stuff.
2.) If you haven't watched the main story first, you should :)
-----*** MY UNDERSTANDING OF THE PLOT ***------
*************HEAVY SPOILER****************
While watching the last episode of Angel Beats you would have wondered, how Kanade ends up in this world before Otonashi.
Here we find out that time in the real world and time in the after world are not related. So it is possible that someone who died 100 years ago could end up here after a person who died yesterday.
This alternate ending (and IMHO should have been the actual one) also answers the question as to who the unknown programmer that created Angel Player and programmed the NPC to turn into shadows actually is.
Wondering who.....? Its Otonashi. In this 2:40 minutes thriller, when the student council president Otonashi leaves the classroom after promising to help a student who realises that there is no point in studying, we hear two girls whispering in the background that he is waiting for someone.
If you remember correctly, (in the main story) the NPC programmed to turn on the shadow patch reveals that the original programmer, was waiting for the one he loved and had turned himself to a NPC.
Or in other words, when everyone disappears, Otonashi decides to stay back and help the newcomers and to wait for his love Kanade. After a while, to preventing himself from turning crazy he turns himself into a NPC ( fully aware that it is possible to turn back into a normal person, as revealed in the main story).
Simply put, this creates a time-loop, where Otonashi is part of the software he himself created.
So this means
1.) Ontonashi is now a NPC.
2.) Otonashi himself was the unknown programmer (reason:time-loop).
3.) Otonashi now waits for Kanade to come back to this world (although the chances are astronomically low)
This is what I understood from this alternating ending :)
And so I give a full 10 on 10.
