String permutations using recursion in Java

Question

I came across THIS post which tried very hard to explain the recursive solution to print all string.

public class Main {
    private static void permutation(String prefix, String str) {
        int n = str.length();
        if (n == 0)
            System.out.println(prefix);
        else {
            for (int i = 0; i < n; i++)
                permutation(prefix + str.charAt(i),
                        str.substring(0, i) + str.substring(i + 1));
        }
    }

    public static void main(String[] args) {
        permutation("", "ABCD");
    }
}

But still I am not able to get the part when we start popping off the stack. For example, recursion goes all the way till permutation("ABCD",""), where base case meets and it prints ABCD. But what happens now? We pop off permutation("ABC","D") from function call stack. What we do with this and so on?

Can someone please help explain a bit?

Also, I need some pointers on the time complexity of this. Not like complete calculation but some hints.