AFAIK it's a pure 2D effect, i.e. no rotation or anything in that regard is involved.
If you want to have angles you will have to use sprites.

If you want to rotate a particle around the "camera-to-particle-axis", for example for smoke sprites or some other effect, you could create a bitmap array which has the particle in let's say 8 or more rotations (made in Photoshop/ ...) and then change the p.bmap pointer dynamically, similar to the sprite+8.tga feature.