ortofon rmg 309 i

Dynamic balancing is not a "thing" you do, it's a design of the tonearm.

Long answer: a static-balanced arm is a simple counterweighted lever.  A dynamically-balanced tonearm still has to be statically balanced (more or less) but uses a spring or method to apply an inertia-damping force to the tonearm in two planes.  In general, static-balanced arms are considered by some to be superior for high-compliance cartridges and dymanically-balanced arms are consider (again, by some) to be of particular advantage to low-compliance cartridges.

Short answer: if your tonearm is a dynamically-balanced arm and it's balanced (VTF is set properly), then it's dynamically balanced.

I apologize for my unclear question,
I am aware that ortofon RMG309 is a dynamically balanced arm,
but I don't understand or don't know how to make it work properly ...
What is the spring used for above the bearing? and the counterweight behind is smooth.? the numbered ring? ..
in short, I need guidance from someone who has this arm and who does it work the best ...
thanks Luigi

Sorry, I misunderstood your question.  I had one of these a few years ago and sold it without ever mounting it.  It's possible I may have made a copy of the manual and if so I will send you a PM. 

ok...very thanks for your help
ciao 

Luigi,
if you still need an answer, the spring is set by the factory default for a combined weight of cartridge and headshell of 31grams. That is with the classic Ortofon SPU headshell. 
When the weight of your headshell and cartridge is 31 g then the numbers on the tonearm weight will correspond to vtf. 
You have two options:
a. Just use a scale to get correct vtf
b. Disassemble the spring and readjust to get a balanced arm based on actual weight of your headshell and cartidge. Instructions are available on line. 
I just use a scale. Easier and arguably more accurate.