Keyword: Reverse Engineering
(yeah I know it's actually 2 words)
I don't play 2Moons/Dekaron (anymore), but I can guess how people do it. First you need to know the most important assembler commands (google for some assembler tutorials). The command "mov" for example copies/"moves" the 2. value into the 1. one.
This line moves the value of eax into edx. Let's say eax is the "dwSight" value of the loaded monster.
If we change the line to that, the value loaded into edx is always 0x10.
As I said before this is just a guess about how it is done.
If you didn't understand something I have written read an assembler tutorial and you will understand.