AutoInvest will deposit funds and then automatically invest them into your pie. You decide 'how much' and 'how often' you'd like to AutoInvest.
You can choose to enable AutoInvest for a pie while you are creating it.
You can also turn on/off the AutoInvest option for any pie at any time, by going to the AutoInvest section of that pie:
How is the newly contributed money allocated?
AutoInvest buys shares according to your pie's targets. For instance, if £100 are deposited into a pie of 5 slices each having a target of 20%, £20 pounds will go towards each slice.