Sensorless BLDC motor control BEMF section

I am trying to make a sensorless bldc motor control driver. I found an Arduino code and I want to convert it ARM Stm32. But I don't understand totally what happens in ISR interrupt part. Can anyone explain me shortly? Why used bldc_step&1 and when decreased i.

byte bldc_step = 0, motor_speed, pin_state;
 
void setup()
{
  DDRD  |= 0xE0;  // configure pins 5, 6 and 7 as outputs
  PORTD  = 0x00;
  DDRB  |= 0x0E;  // configure pins 9, 10 and 11 as outputs
  PORTB  = 0x31;

  pinMode(2, INPUT_PULLUP);
  pinMode(3, INPUT_PULLUP);
  pinMode(4, INPUT_PULLUP);
}
 
// pin change interrupt 2 (PCINT2) ISR
ISR (PCINT2_vect)
{
  if( (PIND & PCMSK2) != pin_state )
    return;
  // BEMF debounce
  for(byte i = 0; i < 20; i++)
  {
    if(bldc_step & 1){
      if(PIND & PCMSK2)     i -= 1;
    }
    else {
      if(!(PIND & PCMSK2))  i -= 1;
    }
  }
 
  bldc_move();
  bldc_step++;
  bldc_step %= 6;
}
 
// BLDC motor commutation function
void bldc_move()
{
  switch(bldc_step)
  {
    case 0:
      AH_BL();
      BEMF_C_FALLING();
      break;
    case 1:
      AH_CL();
      BEMF_B_RISING();
      break;
    case 2:
      BH_CL();
      BEMF_A_FALLING();
      break;
    case 3:
      BH_AL();
      BEMF_C_RISING();
      break;
    case 4:
      CH_AL();
      BEMF_B_FALLING();
      break;
    case 5:
      CH_BL();
      BEMF_A_RISING();
  }
}
  
  PCICR  = 4;  // enable pin change interrupt for pins PCINT23..16 (Arduino 0 to 7)
  
 
 
void BEMF_A_RISING()
{
  PCMSK2 = 0x04;    // enable Arduino pin 2 (PCINT18) interrupt, others are disabled
  pin_state = 0x04;
}
void BEMF_A_FALLING()
{
  PCMSK2 = 0x04;    // enable Arduino pin 2 (PCINT18) interrupt, others are disabled
  pin_state = 0;
}
void BEMF_B_RISING()
{
  PCMSK2 = 0x08;    // enable Arduino pin 3 (PCINT19) interrupt, others are disabled
  pin_state = 0x08;
}
void BEMF_B_FALLING()
{
  PCMSK2 = 0x08;    // enable Arduino pin 3 (PCINT19) interrupt, others are disabled
  pin_state = 0;
}
void BEMF_C_RISING()
{
  PCMSK2 = 0x10;    // enable Arduino pin 4 (PCINT20) interrupt, others are disabled
  pin_state = 0x10;
}
void BEMF_C_FALLING()
{
  PCMSK2 = 0x10;    // enable Arduino pin 4 (PCINT20) interrupt, others are disabled
  pin_state = 0;
}
 
void AH_BL()
{
  PORTD &= ~0xA0;
  PORTD |=  0x40;
  TCCR1A =  0;      // turn pin 11 (OC2A) PWM ON (pin 9 & pin 10 OFF)
  TCCR2A =  0x81;   //
}
void AH_CL()
{
  PORTD &= ~0xC0;
  PORTD |=  0x20;
  TCCR1A =  0;      // turn pin 11 (OC2A) PWM ON (pin 9 & pin 10 OFF)
  TCCR2A =  0x81;   //
}
void BH_CL()
{
  PORTD &= ~0xC0;
  PORTD |=  0x20;
  TCCR2A =  0;       // turn pin 10 (OC1B) PWM ON (pin 9 & pin 11 OFF)
  TCCR1A =  0x21;    //
}
void BH_AL()
{
  PORTD &= ~0x60;
  PORTD |=  0x80;
  TCCR2A =  0;      // turn pin 10 (OC1B) PWM ON (pin 9 & pin 11 OFF)
  TCCR1A =  0x21;   //
}
void CH_AL()
{
  PORTD &= ~0x60;
  PORTD |=  0x80;
  TCCR2A =  0;       // turn pin 9 (OC1A) PWM ON (pin 10 & pin 11 OFF)
  TCCR1A =  0x81;    //
}
void CH_BL()
{
  PORTD &= ~0xA0;
  PORTD |=  0x40;
  TCCR2A =  0;       // turn pin 9 (OC1A) PWM ON (pin 10 & pin 11 OFF)
  TCCR1A =  0x81;    //
}
 
void SET_PWM_DUTY(byte duty)
{
  OCR1A  = duty;  // set pin 9  PWM duty cycle
  OCR1B  = duty;  // set pin 10 PWM duty cycle
  OCR2A  = duty;  // set pin 11 PWM duty cycle
}

This code is for debouncing the back electromagnetic force (BEMF) signal, equivalent code is in https://github.com/esden/open-bldc-mk/blob/master/bldc.c

ISR(ANA_COMP_vect){
unsigned char i;

/* debounce the bemf signal */
for(i=0; i<BEMF_DEBOUNCE_COUNT; i++){
    if(bldc_phase & 1){
        if(BEMF_L) i -= BEMF_DEBOUNCE_DEC;
    }else{
        if(BEMF_H) i -= BEMF_DEBOUNCE_DEC;
    }
}

For debouncing and why it is done see https://en.wikipedia.org/wiki/Switch#Contact_bounce

The exact algorithm implemented here is debouncing switches with vertical counters, cf https://www.compuphase.com/electronics/debouncing.htm - Debouncing switches with vertical counters

In bldc_step & 1 the & operator is bitwise AND i.e. 45 & 35 = 33 or 00101101 & 00100011 = 00100001 in binary bldc_step is a byte and bitwise-ANDing it with 1 is equivalent to testing if the last bit of bldc_step is set. With PIND & PCMSK2 is checked if PIND is high or low

This code and the rest of the debouncing loop implements the vertical counter described in https://www.compuphase.com/electronics/debouncing.htm