You can send well over $20,000 of transfers without your account being closed. They have threatened to close my accounts, as recently as two weeks ago, but for some reason, a promise to reduce activity has been accepted. I would suggest that sending $1,000 per account every four days, particularly back and forth between accounts, will definitely trigger their interest.

As a side note, the other problem they have is that the program effectively enables the user to carry a balance on their Citibank Visa/Mastercard interest-free. Since transfers to the credit card are treated as payments, you can carry a balance as large as the total you transfer from your account to your credit card.