As far as I can tell, they've put together a wide-ranging set of fairly generic-sounding movie music (generic = good, so it doesn't call attention to itself)... but composed it in a way that the three parameters (momentum, depth, power) simply map to different volume levels of the underlying instrumental tracks -- but all perfectly tuned to always produce a pleasing result.
As far as I can tell raising a slider often maps non-linearly to more volume, but sometimes it reduces/removes instruments too.
So there doesn't appear to be any machine learning, nothing crazy fancy -- which is what makes it so clever and impressive in a "I-wish-I'd-thought-of-that" way. I can only imagine the amount of product iteration it took to arrive at what the 3 parameters should be (and why 3 instead of 2 or 10). It reminds me a lot of Adobe's Multiple Masters technology for typefaces. 
If they keep putting out enough of these master tracks, I wonder if it will reduce the market for film composers. After all, most films don't need recognizable motifs, they just need music to tell us how to feel, and this meets that need perfectly. Bad for music students, good for film students...
Not trying to pick a fight with you - I agree with your comment overall, but this is a culturally learned thing, very specific to Western movies and TV-series, and it is actually mildly infuriating to me. It is one thing to complement the emotions present in a scene, but being told what to feel is basically a cheap way to try to patch up bad storytelling.
Worse, it can totally backfire: for the dub of Castle in the Sky, the original composer had to fill in a ton of quiet scenes with music because (IIRC) Disney was worried about the audience feeling awkward about not being told what to feel. They also added background chatter, and worst of all: filled in scenes that were quiet on purpose with all of these things. While the orchestral score sounds great in and of itself, in most scenes all that added audio ruined the mood of the original.
However, I can't deny that what you state is what the audience expects, and that it "works", so sadly you're still right.
Different media are able to show different things -- novels show interior thoughts, plays need to focus mainly on dialog, and movies/TV are largely driven by images.
Particularly, movies and TV have a huge challenge in communicating the interior thoughts of characters -- this is one of the first things you learn in screenwriting, when a particular story might be more appropriate as a novel instead.
But one of the tools which can communicate a character's state of mind is through music. At best, I think it's what you mean when you say complementary -- e.g. a character has to act confident but you need to know that inside they're nervous. But of course it can be a crutch for bad acting too -- if the actor is supposed to be communicating nervousness but isn't, so the music does it for them.
Ironically, animation seems to be a worse offender than regular movies. And sometimes the story itself simply isn't good to begin with, and does not give any decent justification for being emotionally invested in the main characters.
Actually, now that I mention it, Nintendo did the same thing in Super Mario World (1990), just simply adding in some congas to the currently playing track when you jump on Yoshi.
Just regarding your comment about music students, please feel free to get in touch with us if you are interested in writing music for Filmstro. send an email to firstname.lastname@example.org
In fact my favorite game, Rez, has a central conceit that is much closer to this. The heart of Rez is a 32-channel MIDI engine which continuously plays the game's background music; however, which channels are "active" is controlled by a bitmask that varies depending on which phase of the level you're in, allowing for a constantly shifting sohndscape. Then, additional music that arises from game events is mixed on top of that.
Digging deeper in the site does not reduce the copyright ambiguity to me because the example scenarios like a corporate entity picking up content containing the music are dealt with vaguely rather than by pointing to clear licensing terms around copyright. The example of "tracks for a friend" is met with "we don't care." Right now, maybe not. But copyright interest can be sold to someone who does care...and potentially someone who cares because pursuing copyright interests is their business model.
None of which is to say I don't think the tech and music is cool. It just seems that the messy details that can come back to bite an artist sit on the hard work left to do list.
I agree, the FAQ doesn't go into enough licensing detail.
I do like the concept, but I think I'll stick to ccMixter.
As a consumer I always appreciate seeing higher quality production regardless of the product.
Pixabay is a site that works similar to unsplash, but added a video section a short while ago. Not entirely sure if Stocksnap and Pexels have done so too.
Another area where this seems to be happening is with fonts at places like Fontsquirrel and Open Font Library.
We've just created a discount code for you guys. You can use this on any of the monthly subscriptions. It will give you 50% off the first month.
I did discover icons8's free music selection which is actually very good: https://icons8.com/music/
What film have I watched with that in?!!!! It's going to be on my mind all night now... (aargh!)
This music Gnossienne No. 1
is the same as the royalty free one called 'Turin'.
So is it that the Turin version is a royalty free 'recording' (dynamically created), rather than one by a specific performer?
Not bad for consumer mixing... adding music to YouTube wedding videos. Well executed.
That's the way it seems to me as a user anyway, and the way I'd write it as a developer.
Filmstro (https://filmstro.com/music) is the company it's talking about with some seriously cool tech - for every music track, you can control three sliders (momentum, depth, and power) to change the mood of the song in real time - or in Premiere with a plugin.
Of course, this costs money, but it's one of the coolest things I've seen in this field in the last quite a while.
It has got to be a slap in the face to generously offer your art for this use, only to be punished for doing so. Especially with the only way to deal with it is with a tone-deaf automated system.
I'm not saying that's a valid justification, but I imagine it's the thought process.
Then you have to dispute it while they take the ad revenue and let the clock run out.
> If you do a quick Google search for ‘Royalty free music’ or ‘YouTube cleared music’
Yeah, try "creative commons music" instead.
> 100% free for anyone to use – The Filmstro Free Music offering
> The only limitation is that commercial use of the music is not covered with these free tracks.
That's not 100% free.
Also, no download without account.