An Entry Plug is a long, capsule-shaped plug that acts as the cockpit for the pilot of an Evangelion Unit. Judging by how difficult it is to crush one (as seen at the battle against Bardiel, the Thirteenth Angel), its exterior is made of a super-strong alloy that appears even stronger and more secure than the body of an Evangelion Unit.
Its interior is filled with a liquid called L.C.L., which facilitates the neural-link connections between the pilot and the unit (the fluid also directly oxygenates the pilot's lungs). The Entry Plug is then inserted directly into the spinal cord of the Evangelion Unit.
In the human body, the spinal cord and brain make up the central nervous system, and in addition to governing reflex actions, the spinal cord serves as a relay of stimuli transmissions for sensory perception and motion between peripheral nerves and the brain.
By synchronizing with an Evangelion Unit on such a level, the pilot controls the unit as a pseudo-body. Sensory data such as pain is also perceived by the pilot while synchronized. The higher the Sync Ratio, the more accurate control the pilot can exercise over the unit. In an emergency, an Entry Plug can be used as an escape capsule, and is equipped with a parachute and a jettison rockets.