that actually goes for every digit. when you add eight to a number n=10a+b=\overline{ab} (so a is in the tens digit and b is the ones digit), you can think of this as:
- add all of the eight to b
- if b is greater than 10 as a result, subtract 10 from b and add 1 to a
what is 8-10? -2. so if you're shifting the tens digit (a) up by 1, the ones digit is shifting down by 2; if you're looking at the sum of digits, +1-2=-1.
you can do similar things for every other digit added to a number. 8 is -2, 9 is -1, 7 is -3...