Thank you for asking this question and I am not sure how I missed adding this in the lecture. When the subtraction of the DS is negative you add 9 to the negative number.
21 – 14 = 7
3 – 5 = -2 (+9) = 7.
Another example
24 – 7 = 17
6 – 7 = -1 (+9) = 8 (Which is the digit sum of 17 = 1 + 7 = 8)
Thank you for asking this question. Will update the lecture so its more clear. 🙂