Can anyone explain what the purpose of the that little plastic tank left of the radiator tank does? What causes the coolant to flow into there? just curious

 

expansion as the mixture heats up

 

your radiator cap is also a pressure operated valve. it has a spring in it, so when the pressure in your radiator reaches a certian level, it vents some radiator fluid out into the reservour. "high performance" radiator caps usually have a stronger spring in them to keep your cooling system running at a higher pressure.

 

Also of note is that when the engine cools down it draws the coolant back into the radiator thus eliminating air in the cooling system. Water is not compressable, air is so the pressure in the cooling system will stay constant until air displaces the water allowing the air to be compressed building pressure. The spring in the cap releases the excess fluid, (water expands when it is heated) without drawing air back into the system when it cools again.