How does Mutate work?
Play your Sequence (or Arpeggio). Press the Mutate button very briefly, and some notes will change in an unpredictable manner. Or press the Mutate button for longer, and all the notes have been shifted around.
Watch the display while performing this. You'll notice how the Mutate Bar goes from left to right and back again. The position of the Bar, when you release the Mutate button, determines how much Mutate has modified the Pattern.
Notice how Mutate preserves the identity of the Pattern while notes are modified quite heavily.
Note
You can of course press Mutate several times and hear a new variation to your Sequencer Pattern every time.
How to Reset a Mutated Pattern?
If you want to start the Mutate process all over again with an unmodified Pattern, hold Shift and press the Mutate/Reset button. You will be back to your initial pattern, and you can start mutating again.
Holding Shift and sliding up and down the Modulation Strip will add random changes to the current Sequence. The further up the Modulation Strip you go, the greater the changes. Spice adds randomness to Gate Length and Ratchet (adding multiple triggering).
The Mutate function generates a new pattern for Spice. Alternatively, Shift + 3 taps on the Mod strip also generates a new Spice Pattern but without adding mutations.