The short version, with nothing hidden
Every music app plays you more of what you already like. Slippy is built to do the opposite.
Slippy does not listen to a record and look for another that sounds like it. It looks at what real listeners play side by side. If thousands of people who play the song you named also play something else, that something else is worth hearing next.
This is why Slippy turns up bands that no genre list would put anywhere near you. People's listening does not respect genre, and neither does the trail it leaves.
Most apps want to keep you comfortable, so they play you the song by that band you were always going to find anyway. Slippy aims at the middle of whatever corner of music it is standing in. Start it on a huge song and it stays around big songs. Start it somewhere quiet and it stays quiet.
It also holds back the two or three songs every band is famous for, because you have almost certainly already heard those.
Anything from the playlist you gave Slippy is never played. You already have it, and that is the whole point.
Anything you throw out never comes back either, and throwing out two by the same band means Slippy leaves that band alone.
Everything else is rested rather than banned. A song the radio found will not turn up again for a long stretch of listening, but months later it can, the way it would on any radio. Press the heart and it can come back whenever it fits, because you said you liked it.
One song is a guess. Slippy has a single thing to go on and has to work outward from it.
Hand it a playlist and it has hundreds of songs to learn from instead. It works out which bands matter most to you by how many of their songs you keep, reaches back into your own music every few songs so it never wanders off, and refuses to play anything you already own. Every song it finds tells you which of yours it came from.
A mood is not a kind of music. Nobody labels records "driving at night", but everybody knows what it sounds like. Slippy works out which artists belong to a moment like that, starts there, and keeps checking back so an hour later you are still in the same mood rather than somewhere else entirely.
Whole songs play in YouTube's own player, so any advert is YouTube's. That is how the musician gets paid for the play, and Slippy leaves it alone.
Paying for Slippy does not remove them. YouTube Premium does.
Two dials sit under the player. One sets how well known the songs should be, from hardly known to well known. The other sets how far it wanders from the song playing. Turn either and the change is heard on the very next song.
Slippy never reads your listening history. It knows only what you hand it: a song you name, a playlist you give it, a mood you ask for. Nothing you play by accident counts, so one strange evening cannot spoil what it plays you for months afterwards.
It is also not tied to where your music lives. Bring a playlist from anywhere, and take what Slippy finds anywhere you like.