This technique can be used when one of the numbers being added is very close to an even ten, hundred, thousands, etc. boundary. For example, 398 is just 2 less than 400.
This fundamental technique can be used in special cases to add numbers large and small. Once mastered it will help speed up all other trick calculations.
Example
586 + 398 = 984
Step 1
Identify that one of the numbers is close to an even number and calculate the difference.
398 is just 2 less than 400.
Step 2
Use the rounded number in the original calculation to make it much easier.
586 + 400 = 986
Step 3
Subtract the result found in step 1 from the result in step 2.
986 - 2 = 984
The technique can be modified and used for a number that is slightly larger than a rounded number. For example, 402 is just 2 larger than 400. In this case the difference would be added back in step 3.
This technique, while simple, can be much faster than technique #1 or long addition for the cases in which it applies.