[SV650rules]

The thermostat keeps the water out of the radiator and shortens the cooling loop to just water pump and cylinders to allow it to warm up faster, when water reaches a certain temp the stat opens and allows water into radiator, if water gets too hot the fan will kick in. Venting hot water to atmosphere is not done (unless you remove radiator cap when engine is hot, which releases pressure and lowers boiling point (just like kettle boils at lower temp up a mountain) one of the things coolant additives do is raise the boiling point to something like 120degC or more, because last thing you want is water boiling in a cooling system. Any expanding coolant from cooling system is routed under rad cap relief valve into expansion tank, when system cools the coolant is sucked back into main system. Cars and bikes work the same in this respect.

Venting hot coolant to atmosphere on a bike could easily result in a severely scalded right knee, but no modern system vents to atmosphere anyway, the cap seals on rad, the spring loaded valve only allows coolant access to expansion tank.