Alien stealth technology relies on the interception and absorption of electromagnetic waves. While the aliens do not use radar absorbent materials in the human fashion, UFO hulls are designed to conduct an electromagnetic interference field. This field jams, distorts and scrambles electromagnetic waves, making conventional target acquisition via radar or even the naked eye problematic. Note that this does not turn the UFO invisible in any way. Rather, it distorts its appearance. A UFO can thus appear "wavy" or "mirage-like" or it can look like a "streak" or a "pulsing dot" of varying size and/or intensity.
The use of an Elerium-115 Reactor for propulsion in a UFO has additional benefits in the form of a sort of "shield" system. This special feature of the hull is due to its unique construction, wherein the hull of the UFO is used to conduct, amplify, and shape the gravity waves produced in the Elerium Reactor. Looked at as a cross section, the gravity field produced by a UFO resembles the magnetic field of a planet, with the exception that a UFO produces both an "external" and "internal" field. The internal field is also called the inverted or reciprocal field. This internal field keeps the ship gravity neutral and seemingly inertia-less, even as the external field can bloom to hundreds of times normal gravity as it pulls the ship in a given direction.
A side effect of the external gravity field (which expands and contracts in different directions as the ship maneuvers) is that it forms a donut shaped shield bubble capable of redirecting projectiles, or overpressure waves. The gravity shield produced by the propulsion system in this way is notably less effective against laser or plasma weaponry.