So say we want to round 7423 up to 10,000. The amount you rounded up by is 2577. Below is the breakdown of the steps.
7 + 2 = 9
4 + 5 = 9
2 + 7 = 9
3 + 7 = 10
The first digits in the above addition 7,4,2,3 are the digits of the original number (7423).
The second digits in the above addition 2,5,7,7 is the amount you have rounded up by (2577).
If instead you want to round up to 8000 instead of 10,000 you dont do the first addition. (7+2 =9). So the amount you rounded up by will be 577 (the last three digits of 2577 when we rounded up to 10.000)
I showed you the calculation step by step just for your understanding. If you do 10,000 – 7423, then you are explicitly calculating the numbers.
If you know that the last digit adds up to 10 and the other digits add up to 9 you will be able to tell the rounded up value without calculating anything. So you can immediately see the number 7423 and say rounded up number is 2577 without doing any calculation.
Hope this clarifies. Let me know if I understood your question correctly and answered it.