inside which the Evangelion
pilot sits. The Eva
is activated by inserting this plug
into its cervical vertebrae
(the same applies to the simulated entry plug, the dummy plug
). In an emergency
the plug also serves as an escape capsule
, and is equipped with a parachute
and ejection rockets
for this purpose.
Source: The End of Evangelion : Glossary contained within The End of Evangelion - Theatrical Program.
The Entry Plug is the tool with which Nerv control the Evangelions. The Evas's actions are ultimately dictated by their resident souls (in Eva-01's case, Shinji's mother is the resident soul) however the resident soul of an Eva can be interfaced with via the pilot, who resides within the Entry Plug. I have seen production sketches which show a cross-section of the Eva's Core (where the Eva's resident soul resides) with the Entry Plug being buried inside. The Entry plug is also known as the Thrown of the Soul, and acts as conduit between the pilot and the Eva's soul. At one point Asuka describes the Entry Plug as like being in a womb. It's never explained exactly how this interface works, however it is clear that it an important part to the operation of the Evas as a fighting unit.