Practising a new song at home meant playing along with the whole mix, your own part included. Now every song is split into four stems, drums, bass, vocals and everything else, so you can take yourself out of the mix and play against the rest of the band.
Open a song in the studio and tap Stems at the top left of the waveform. The four lanes replace the waveform at the same size, each with its own small waveform, a mute and a solo button, and on a computer a level fader. Mute your own lane, or solo the one you want to hear, and the song keeps playing from where you were. Full mix puts everything back, and Mix returns you to the waveform. Each stem can also be downloaded from the download menu for practising elsewhere.
The split is made automatically when a version is uploaded, and the songs already in your bands have theirs. It is not a studio-grade separation: the "other" lane holds guitars, keys and whatever is left, and a little of one instrument can bleed into another. For rehearsing a part it does the job.
On a phone the stems take a moment to load the first time you open them; the mix keeps playing until all four are ready.