I have seen a lot of you guys talking about it. Can someone explain that? 

never heard of a water leakdown test,are you talking about a cylinder leakdown test?

You use a tester that replaces your radiator cap and you pressurize the system by pumping in air. The tester has a gauge on it so you can tell whether your loosing coolant or not.

 

Thats cool. I didn't know those exist. I always just checked and looked for water leaks. Or even steam.

sometimes that will work.  Sometimes leaks only happen when hot, sometimes only when cold.  It won't find the tiny pinhole head gasket beginning failure.

 

But it will make some ordinary leaks easier to find.

Ohhhhhh. That makes sense. Thx

most  small head gasket leaks will happen only  at full operating  temperature  as  Subaru  nut says