...and adding the base to the top digit of this position before performing the subtraction.
In base 10, it means you decrement the top digit to the left of the current column, then add 10 to the top value in the current column.
For example, in the following subtraction:
21
-19
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In order to subtract in the ones column, I'd have to borrow from the tens column, so I'd decrement the tens column and add 10 to the 1 in the top ones column. It's probably more familiar if you see it written out like:
1 11
-1 9
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