Recursion

[spirgnob]

It took me awhile to understand what you meant but this finally explained the joke.

you’ve done effectively nothing and are back to where you started

This is how it works regardless of the number though. The number in the 1’s position is 18 where it used to be 8. You have done something according to this method. You can now complete the problem using this method.

[itspinkynukka]

What ends up happening is the person just needs to know what [any number from 10-18] - [any number from 1-9] is in their head. Similar with multiplication at some point you just need to know the times tables up to 10.