Does anyone know about flywheel counterweight? How does counterweight works? I'm thinking about geting whole kit.

 

The stock manual flywheel has it built in to it, when you change it to a lightweight you need to use the auto counter weight to account for the lightweight not having the counterweight.
They are like $100

 

The counter weights are there to offset the lobes on the eccentric shaft. There is one on the front, inside the front cover, and the other one is cast onto the outer edge of the stock flywheel.

 

When replacing to a light weight flywheel, after market manufacturers usually makes the flywheel without the counter weight cast on to them.

The automatic transmission uses similar design, and have the counter weight bolted on towards the inside of the flywheel.

With the combination of lighter material, and counter weight moved inwards of the flywheel, the rotational mass will decrease dramatically, less inertia, and improves the responsiveness when reving the engine.