The feature was developed as a 20% project by Google developer two allow people to have 3D support inbuilt in YouTube, so that a single video could be uploaded by the creator for the different 3D video formats that are out there.
The feature is currently in beta status and has as such not been documented anywhere. However it can be activated and controlled applying using special tags to your videos (as explained by the developer in this forum discussion):
yt3d:enable=true Enables the view mode. (obviously you've already discovered this) yt3d:aspect=3:4 Sets the aspect of the encoded video. yt3d:swap=true Swaps the left and right sources. You may need to add this to videos when the player with fixed anaglyph modes ships. Apologies for the inconvineince. yt3d:left=0_0.1_0.5_0.9 and yt3d:right=0.5_0.1_1_0.9 These tags are very provisional and most useful for fixing up old videos. They set the source area for each eye as pairs of coordinates x1_y1_x2_y2. The scale of these coordinates is 0,0 for the the top left down to 1,1 for the bottom right.