Another explanation:

Your finger points to the first element. The index decides how many places you shift your finger to the right. If the number is negative, you shift your finger to the left.

Of course, you cant step to the left from the first element, so the first step to the left wraps around to the last element.

Another explanation:

Your finger points to the first element. The index decides how many places you shift your finger to the right. If the number is negative, you shift your finger to the left.

Of course, you can't step to the left from the first element, so the first step to the left wraps around to the last element.