Meridian introduces Sweep | Meridian Credit Union | Discussion forum

Banks want you to keep as much as possible in the chequing account (reduces interest costs), and fear of insufficient funds is one of the incentives, which is why you haven't had auto transfer from savings to cover a chequing account.

There's no details on exactly how Sweep works, but it sounds like you can set it to say "keep a minimum of $XXXX and a maximum of $YYYY in the chequing account, transferring to and from the savings". But they also mention their overdraft service, so I'd assume if a big debit went through and made you NSF, the overdraft would have to kick in first, then Sweep might come along after to top up the account. Overdraft is $1/month, it seems. Perhaps they just mean that overdraft would kick in only if your savings account also couldn't cover things, but one would have to see exactly what mechanisms Sweep can be set up with.

Ideally, there would be a single account, which pays chequing rates (i.e. nothing) on the first $ZZZZ of the balance, and the savings rate on amounts above that. But even accounts where fees are waived at a certain minimum balance always have more than that in it, so banks would have to set the amounts higher than the current values to break even.