Help me understand this cool/short code on OpenProcessing

Processing Coding Questions
1

I found the following code on Open Processing.

https://www.openprocessing.org/sketch/374455

Would somebody help me understand the Particle class? In particular, what is the math going on in the update function?

class Particle{
  
  PVector pos, vel;
  float saturation;
  
  Particle(){
    float posAng = random(TWO_PI);
    float posSize = random(maxRadious);
    pos = new PVector(posSize * cos(posAng), posSize * sin(posAng));
    float velAng = random(TWO_PI);
    float velSize = random(minVel, maxVel);
    vel = new PVector(velSize * cos(velAng), velSize * sin(velAng));
    saturation = random(100);
  }
  
  void update(){
    pos.add(vel);
    if(pos.mag() > maxRadious){
      pos.limit(maxRadious);
      float posAng = atan2(pos.y, pos.x);
      float velAng = atan2(vel.y, vel.x);
      float velSize = vel.mag();
      vel.x = velSize * cos(2 * posAng - velAng - PI);
      vel.y = velSize * sin(2 * posAng - velAng - PI);
    }
  }
}