We don't know exactly since Windows Media Player is not our product But we offer the following explanation.
There are two types of indexes in .avi files: old (idx1) and new (indx). New indexes are being written before the frames in memory, while old indexes are being placed to the very end of memory (behind all frames). Therefore .avi files with "idx1" can be played only when downloaded completely, while .avi's with "indx" can be played immediately.
VideoCharge allows to create .avi files with "idx1", "indx", both or none type of indexes.