Based on the design of the head, angle of the engine etc....the back corner (driver's side) has a pocket of basically steam. It has been determined that especially on higher compression or turbo cars, this is a weakness that can cause premature failure of the head gasket due to excessive and inconsistent heat across the head.
You drill and tap a hole in the back side of the head, tie a small hose into that elbow fitting, route that up to the small return hose on the radiator return and voila you allow an additional passage for water flow which eliminates the pocket that potentially never sees proper cooling.
Linsdey sells a kit but you can buy everything you need at the hardware store to do this.
This is best done when the head is off the car or engine is out.