The Brain is where you can adjust how Plum will set money aside for you, according to your preferences.
This is where you'll find the Rules:
Automatic
Round-ups
Pay Day
Rainy Days
52-week Challenge
Also, it's the place to set if you'd like Plum to deposit for you in Overdraft by tapping on the Overdraft Deposits menu.
Finally, you can find the Splitter, which is a feature that allows you to, well, split the auto-deposits made by Plum to various Pockets and/or Investments funds.