Ensures() - guideline support library

I am trying to understand how to use Ensures() in code. As given in the example, if I tried using Ensures() as follows...

int main(void)
{
    int result = 0;
    // Some calculation
    Ensures(result == 255);
    return 0;
}

If the result variable is not equal to 255, the program crashes with the following output "terminate called without an active exception". My question is how to use Ensures() properly?