Finding the value needed

Because I am learning coding I was trying to figure out why bases use there code and how they found it.
So I was checking the ReloadAnimratio and I found these 3 lines:

MOV ECX,DWORD PTR DS:[10DB1694] This is the weapon pointer or whatever you call it.
LEA EDI,DWORD PTR DS:[ECX+ESI*4] Here I can see what I see in bases posted on MPGH, I just don't have a clue of the value of ESI. I hope somebody could explain this to me.
MOV EAX,DWORD PTR DS:[EDI]
FSTP DWORD PTR DS:[EAX+C3C] Here the offset got added

So I have 2 questions:
1. What is the ESI register doing and what is it value?
2. How do people found out that the no reload value has to be 100 for no reload?

Is there no one that can help me? :/