1. Possible Tritone Sub
One functional harmony explanation is: this a deceptive cadence to a minor iv chord. Working in
Cmin, it would be fairly common to see these sorts of progressions:
| Cmin | Cmin | C7(♭9,♭5) | Fmin |
| Cmin | Cmin | F♯7 | Fmin |
As you know (and as the two lines above demonstrate), the F♯7 chord is a tritone substitution for C7alt, and the F♯7 chord wants to resolve to Fmin. When I listen to the songs you linked, I find myself waiting for the minor iv chord to appear. The fact that we move back to the minor i chord (Cmin in the example above) instead of the minor iv chord (Fmin) is surprising.
The progression above doesn't have to have a minor tonality. In other words, it may not be a imin-i7alt-ivmin scenario. We could have something like this:
| Cmin | C7(♭9,♭5) | Fmin | B♭7 | E♭Maj |
| Cmin | F♯7 | Fmin | B♭7 | E♭Maj |
2. Intentional Dissonance/Switching Tonal Centers
The analysis above is a probably bad fit for scenarios where the progression repeats. In the songs you've cited, the progression continues to cycle back and forth between the imin chord and the ♯IVMaj chord. This can indicate a shifting tonal center. After all, Cmin and F♯Maj represent possibly the biggest possible tonal shift that can occur. For one, the roots of Cmin and F♯Maj are a tritone apart, classified as a dissonant interval and generally considered the most dissonant. Secondly, the change from a minor chord quality to a major chord quality represents an additional shift that catches the ear.
(As a note: the F♯Maj and Cmin scales have more common tones than F♯min and Cmin. In terms of key signatures, F♯min is "farther away" from Cmin than F♯Maj is. But Cmin → F♯Maj can still sound like more of a change than Cmin → F♯min because preserving a minor chord quality creates continuity. So when moving the root by a tritone, changing from a minor tonality to a major tonality is arguably a bigger shift than maintaining a minor tonality. The song feels oddly uplifting at the ♯IV chord. This can contribute to the feeling that we're moving between two tonal centers.)
Some of the links you included contain additional evidence of shifting tonal centers. For example, in the last link you shared contains this progression:
| Amin | E♭Maj | Amin | B♭Maj |
It's almost like we're hearing a I-V progression in E♭, but with an A in the bass. This sort of technique--playing related major chords on top of an unrelated minor chord--is not that uncommon in certain types of avant garde music. The desired effect is one of dissonance, and I think you're finding that this element has made its way into some heavy metal music.
3. Phrygian-Like Progression
Another way to think of the progression is that it stems from C phrygian. To see this, we can first consider this very common progression:
| Cmin | D♭Maj | E♭Maj | D♭Maj | Cmin |.
C phrygian scale is an extremely common choice for this entire progression. But if we add the Cmin blues note (G♭), then now we're very close to a G♭Maj chord. In fact, the G♭Maj chord might sound very natural in this progression given the presence of the D♭Maj chord, and the G♭ can have the effect of adding a bluesy feel to a phrygian mood. This explanation provides a way to look at the ♯IV chord as a part of the existing tonal center.
4. Diminished Sound
Another place we might look, when explaining the ♯IV chord, is the diminished scale. In fact, the chord G♭Maj/C is a common voicing for C diminished. A song that switches between Cmin and G♭Maj/C might simply be an example of modal interchange, where the progression is
| Cmin | Cdim/G♭ | Cmin | Cdim/G♭ |. One could play C half-whole diminished continuously over this entire progression.
While this is a good explanation for a imin-♯IVMaj progression, it doesn't quite fit with the examples you've provided. In your examples, many of the melodies using the full minor scale followed by the full major scale. I don't think there are any where we hear an actual octatonic scale (or even the same scale) over both chords.
5. Minor Blues Feel
The final option we can explore is a i → ♯IV(♭5) progression. I don't believe the specific recordings you shared fall under this category, but it is an interesting possibility.
To consider a ♯IV(♭5), we might imagine the progression:
| Cmin | F♯Maj | Cmin |
as being something more like:
| Cmin | Cmin7(♭5)/F♯ | Cmin |
Many pianists will use Cmin7(♭5) as a voicing for a bluesy Cmin chord. Putting the tritone in the bass simply adds some heightened dissonance/temporary tension, which releases on return to the
Cmin7 chord. If we ignore the root, the voicing can be identical to an
A♭7 chord. So we can take the i-♯IV(♭5) progression and characterize it like this:
| Cmin | A♭7/G♭ | Cmin | A♭7/G♭ |
In this way, we treat the ♯IV(♭5) chord as a slash chord: ♭VI7/♯IV (A♭7/G♭ in the example above). The i-♭VI7 progression is extremely common in many forms of jazz, so moving the bass down a whole step from A♭ to G♭ can be viewed as a way to add some variety/dissonance.