It is simply a chromatically altered chord. Many Hookpad users confuse them for applied dominant or simple mixture (in Minor it is, but it may be seen as a product of melodic minor).
Major II - IV and minor IV - VI produce a ♯4 - ♮4 [resp. ♯6 - ♮6] voice leading, hence virtually all common practice theory books avoid this progression since it is against the rules for using accidentals. Major IV - ♭VI uses the chromatic submediant, but at times the ♭VI chord could be replaced by an augmented sixth chord.
ZUN uses IV6 - VI and the ii46 - VI variant a lot.
Yeah, kinda what HertzDevil said. 6# goes to a 6 natural. Sounds really good.
You can see chromatic stepwise motion in a lot of other chord “pairs” or “sequences”. IV -> II -> V -> III -> vi. IV -> iv. I -> I7 -> I7b. I guess II -> IV is just another part of the IV -> II -> V -> III -> vi “bridge”, but going the opposite direction.
Also, II makes a really good pre-cadence chord. It kind of makes it feel like the section is “wrapping up”. It’s not bad to use a II to set up a V or a IV -> V, like in the Violet City Example.
Any book recommendations which avoid “basic” assumptions and rules of theory? I really feel like a lot of the music I very much enjoy lately is a complete departure from what anyone who learned too much theory before writing their own music would come up with. For instance Flosstradamus - Soundclash and Knife Party - Boss Mode are both very … odd, to me. There’s no Soundclash tab but it sounds to me like it’s a chromatic scaled melody against a major scale. (I tend to have errors in my analysis though)