"Anti"-gravity is really a misnomer but one I sometimes use because it's a word people are familiar with. What these newer technologies due is create a bubble of sorts around a craft and this bubble is not affected by gravity due to the way it polarizes virtual particles in the quantum vacuum.

So, it's not so much that one is repelling gravity or acting against it, but rather that one is reducing the effect of gravity to zero or near-zero in a localized space.

A bit of background on Thomas Brown, a U.S. Navy engineer who did some of the early work in this field: http://jnaudin.free.fr/html/ttbrown-01.htm

A videotaped lecture by Paul LaViolette, who covers some of the more recent developments in this field: https://www.youtube.com/watch?v=ifEgGMFK-VU