Reminds me of using a slide rule. You normally push the inner part (the C scale) to the right, line up the 1 on the C scale with the first number you're multiplying on the D scale, then look on the C scale for the second number you're multiplying, and read the result off the D scale immediately below that.
But when the result is more than 10, you've wrapped: your answer is off the D scale. So now you have to push the inner part back to the left, and line up the 10 (usually marked as 1, at the right-hand end) on the C scale with the first number on the D scale. And remember to add 1 to the exponent.
I've seen slide rules where the D scale goes slightly beyond 10 (like 10.1), so if the result was just a tiny bit over 10, you wouldn't need to wrap.
