This is also known as E Mechanism. It is an automatic mechanism (That is the player doesn't need to alter fingerings) that holds down the lower G key when fingering third octave E. It improves the venting for this note and therefore, makes it easier for most players. It is usually recommended for beginner flautists to have this, but for the more advanced player it can make a G to A trill in the third octave more awkward. It is also worth noting that an E mechanism can cause problems on an 'In-Line' open holed flute unless it has a 'Pinless' mechanism. On normal offset G flutes (both closed and open hole) there is no mechanical problem associated with E mechanism.
An Example of Split E Mechanism