Overlay: what's it about, and why doesn't it work sometimes?

[Juggalo]

Hi,

Can anybody explain what the "overlay" feature does, and what is the difference between mode 1 and 2?

Also, I noticed that sometimes BsPlayer "fails to create overlay", yet other times it works fine (i use v0.86 build 493). In case it has to do with hardware: I have a GeForce4 Ti4200 128Mb using NVidia's Detonator 30.82 driver (and shitloads of free ram).

I even noticed that some movies won't play if I disable this Overlay, or only play in mode 1 or 2. Why is that? Does overlay have anything to do with decoding a movie??