As described by a Reptiloid Alien who goes by the name of "Lacerta" describes the force field effect that causes the gravitational shift, which is used by most alien saucer craft for propulsion.

"The object itself is metallic and disk-shaped; certainly it is distorted in form and color by means of a field effect. These four white and very long "processes" on the underside of the ship itself portray a kind of quasi-gravitational light manipulation, i.e., the universal force field is being shifted in the direction of a simulated gravity. Actually, it is not a genuine light (it is mostly not a genuine light whenever you see illuminating "UFOs") but a special strongly charged form field which manifests itself in the space that matter inhabits as a quasi-light. The reason for the activation of this special high-energy system in an atmosphere is not completely clear to me; it's possible that it is a kind of investigating or influencing of the environment."

"By means of the fluctuation at the right angle with the induced radiation field, copper is fused with other elements. (The illusion of matter is fused, the fields in the sphere of influence overlap each other, but the main force would be reflected by that process and would assume a quasi-bipolar character.) The resulting connection and the field would therefore not be stable in the normal condition of matter and unsuited for tasks. As a result, the entire field spectrum is shifted to a higher plasma-like condition, whereby the spectrum comes together with this harsh shifting to the opposite pole side—the word is NOT correct—of the force field and it resembles quite closely a gravitational shift. This shifting causes a "tilting" of the repulsing quasi-bipolar force, which now no longer flows to the interior of the force field, but rather flows partly to the exterior of the field. The result is an inter-stratifying reflective force field which is very difficult to modulate within certain technical boundaries in relation to its own characteristics. It can also carry out a multiplicity of tasks, as for example, causing massive flying objects to be levitated and maneuvered. It can also exert a camouflage function in the realm of electromagnetic radiation as well as manipulate temporal sequences of events—indeed only to very limited extent—and other things as well. Are you familiar with your "quantum tunnel effect"? Even the amplitude equalizations among genuine matter can be achieved with one of those kinds of fields if the frequency and the distance from the plane of the field are high enough." Quoted from the interview with "Lacerta" The reptiloid female who lives underground on Earth with the rest of her species.

You can read the entire Lacerta interview here.

Lacerta Reptiloid Alien Interview