The servo can't move back and forward in 40 milliseconds. It'll be more like tremble between both.

Try something like this:

// keep sending 0 position for about 400ms
for(int i = 0; i<20; ++i) {
  digitalWrite(PB1, HIGH);
  delayMicroseconds(1000);
  digitalWrite(PB1, LOW);
  delay(19);
}

// keep sending second position for about 400ms
for(int i = 0; i<20; ++i) {
  digitalWrite(PB1, HIGH);
  delayMicroseconds(2000);
  digitalWrite(PB1, LOW);
  delay(19);
}

The servo can't move back and forward in 40 milliseconds. It'll be more like tremble between both.

Try something like this:

// keep sending 0 position for about 400ms
for(int i = 0; i<20; ++i) {
  digitalWrite(PB1, HIGH);
  delayMicroseconds(1000);
  digitalWrite(PB1, LOW);
  delay(19);
}

// keep sending second position for about 400ms
for(int i = 0; i<20; ++i) {
  digitalWrite(PB1, HIGH);
  delayMicroseconds(2000);
  digitalWrite(PB1, LOW);
  delay(18);
}