I gather from VOY "Flashback" that they are simply the same as regular shields, but either rapidly tuneable to multiple phases, or then able to utilize multliple phases at the same times. Kirk never tuned his shields to match an opponent's attack, supposedly because his hardware wasn't up to it, and possibly because the science of his day hadn't even figured out yet that tuning might be beneficial.
So, what is a "phase"? It's unlikely to be the same as "frequency", or else there would be no need for two separate words. But if and when shields have a frequency (and perhaps also a frequency for the changing/"rotating" of frequency, possibly considered metafrequency), they appear to be described as waves of some sort, and waves also have a phase, i.e. a specific placement for the wavetops where frequency is the (inverse of the) distance between the wavetops.
From "The Next Phase" and "Time's Arrow" we learn that all things in Trek have a "phase", and nudging that makes the out-of-phase objects less tangible to those left behind in the normal-phase universe. Phasers also make things less tangible, sometimes 100% so! And transporters turn people into "phased matter streams" that are very intangible, going through walls and empty space with equal ease.
Perhaps this "universal phase" of Star Trek is what is being adjusted in the shields, too? Perhaps it's possible to make things more tangible than regular matter, thus creating impenetrable obstacles? Or perhaps various weapons (such as phasers!) exploit a specific band of phasedness to get past obstacles, and shields are configured to countermand that specific phase; multiphasic shields can handle several phases at a time, against multiple enemies or enemies who rotate their weapon phases; and metaphasic shields have a "phase of phase" of some sort.
Such an interpretation makes "phase" and "frequency" sound interchangeable in practice. But they need not be that in the Trek reality: weapons and shields may have both qualities, and while the latter is something the heroes and villains have learned to adjust with relative ease in the 24th century, the former is still inflexible enough that going transparent or blocking phase-ghosts are still stunning and tactically revolutionary techniques.
Timo Saloniemi
