What exactly is PINB | = 0b00000001;

Projects General Guidance
18

Using the toggle feature of PINB is not safe. If any toggle gets missed or repeated, your PWM is suddenly inverted.

  elapsedMicros = micros() - previousMicros;
  
  if (highLow)
  {
    // HIGH
    if (elapsedMicros >= highTime)
    {
      previousMicros += highTime;
      PORTB &= ~_BV(PB0);   // Clear Pin 8
      highLow = 0; // started low period
    }
  }
  else
  {
    // LOW
    if (elapsedMicros >= lowTime)
    {
      previousMicros += lowTime;
      PORTB |= _BV(PB0);  // Set Pin 8
      highLow = 1; // started HIGH period
    }
  }