Twilight psytrance is a term first used by South African artists Shift and Pitch Hikers to describe their music and the whole SA scene, since the term 'South African psy' doesn't sound that correct (many artists that produce music to this kind are not from SA).
Twilight psy is a harder and darker version of Full On that also contains non cheesy melodies. Hence the music is suitable for both day and night, and even better for twilight hours.
IMO this music although it's closer to the Full On genre it has so many differencies that correctly the artists use a different term.
Another similar terms are nightpsy (just like darkpsy, but it's focus in night and not darkness) or hardpsy.
Many use this term to describe artists that are close to the SA sound, but not that phat.
Like Abomination, Azax Syndrom (mostly older tracks), Seroxat etc.
Invest in Twilight Psy without fear. Most of this music is awesome!
Here is a 'beginners guide' to SA albums.
Check also albums by Phatmatix, Tryon, Sidhartha, Wizack Twizack, Dark Nebula.