M1 buys securities in several situations:

• when the cash balance in your account is above your cash control threshold
• Your available cash balance will increase when making a cash deposit, receiving dividends, or selling securities.
• Initiating a rebalance
•  You can also use cash to make orders to buy individual securities or slices of your Pie. 

M1 sells securities in several situations:

• Initiating a rebalance
• Requesting a withdrawal
• Removing a security from your portfolio
• Making orders to sell individual securities or slices of your Pie.