关于minifly yaw轴pid控制的问题

AGopendv

if (RATE_DO_EXECUTE(ATTITUDE_UPDATE_RATE, tick)) /* 500Hz */  
{
  if(setpoint->isAltHold == false)
  {
   actualThrust = setpoint->thrust;
  }
  if(control->flipDir == CENTER)
  {
   attitudeDesired.yaw -= setpoint->attitude.yaw/ATTITUDE_UPDATE_RATE;
   while(attitudeDesired.yaw > 180.0f)
    attitudeDesired.yaw -= 360.0f;
   while(attitudeDesired.yaw < -180.0f)
    attitudeDesired.yaw += 360.0f;
  }
  attitudeDesired.roll = setpoint->attitude.roll;  
  attitudeDesired.pitch = setpoint->attitude.pitch;
  
  attitudeAnglePID(&state->attitude, &attitudeDesired, &rateDesired);
  
  if(control->flipDir != CENTER)
  {
   rateDesired.pitch = setpoint->attitude.pitch;
   rateDesired.roll = setpoint->attitude.roll;
  }
  attitudeRatePID(&sensors->gyro, &rateDesired, control);   

不太明白怎么实现的yaw轴转动速率的控制，尤其红色部分第一行，求解答！