Pressing 'P' will allow you to edit the following parameters for the currently selected particle style using the cursor keys:
Brush Size: The webcam image is replicated using lots of little brushes that move about the screen. This controls their size.
Attraction: How strongly the brushes are attracted to movement in the webcam image.
Persistence: This controls how long registered motion affects brush movement. Probably best to just play with it to see the subtle effect it has.
Friction: How quickly brushes slow down.
Col Attack: How quickly brushes acquire the colour of the background pixel when moving over an area of motion.
Col Decay: How quickly brushes acquire the colour of the background pixel when moving over a static area.
Brush Repel: How strongly brushes repel each other.
Collide Damping: Controls the elasticity of brush collisions.
Brush Number: Sets the number of brushes.
Velocity Match: Controls how much brushes affect the velocity of nearby brushes.
Pressing the P key again will remove the edit display info from the screen.