Doubt with a test of a Java program

I have the following question about my Fundamentals of Programming class, the professor gives us the following code:

import java.util.Arrays; 

public class blablabla {

public static void main(String[] args) {
    int[] v1 = new int[] {9,6,2,1,4,0};
    int j;

    for (int i=1; i<v1.length; i++){

        int temp=v1[i];
        j=i-1;
        while (j>=0 && v1[j] > temp){
            v1[j+1] = v1[j];
            j--;
//                System.out.println("I es:" + i);
//                System.out.println("J es:" + j);
//                System.out.println("Temp es:" + temp);
        }
        v1[j+1]=temp;
//            System.out.println(Arrays.toString(v1));
    }

}
}

To this code you had to do a desktop test (a paper and pencil) to organize the array from least to greatest, so my question is, why does the program start with j = -1 when i = 1 and achieve Arrange the array correctly? Since the teacher when doing the test on a desktop sheet says that J = 0 when i = 1 and following the program line by line arranges the array correctly.