@David Dachtera, @Chris May, @Joe Au
I can speak about the origins of this concept and why it works.
This whole theory is based on a specific type of mortgage that exists prevalently in Australia and sparingly in the UK. I know it well because I had such a mortgage in the UK.
https://en.wikipedia.org/wiki/The_One_account
In the UK they called it the One Account and basically it is a combination of mortgage, checking account and line of credit - all in one. You buy a house for $100k with 20k down. From then on you have an "available facility" of $80k - that facility will have a certain timeline (20-30 years) before it expires. You use the account as a checking account so that your income and expenses come out of it and you pay a set interest daily on whatever you owe. So, if you receive $5000 in income you are now paying interest on only $75k. A few days later you pay a bill for $1,000 and now you are paying interest on $74k. The interest is calculated daily but paid once monthly.
So yes, you can save some money either by:
1. if you take advantage of interest free offers, credit cards with 60 days to pay and just the lag between income and expenses you can save by basically "earning" the same interest rate on the mortgage on any money you can keep even for one day in the account.
2. if you need money for another purchase such as a car or whatever, you can withdraw the money up to the facility and only pay interest at the mortgage rate (typically lower than other rates)
I recently sold the apartment in the UK that was under this mortgage and all that happened was that once I transferred the sale money, the account now had a positive balance and so I no longer owe any interest.
It seems that someone has tried to reverse engineer that concept into the US reality by using separate HELOC, mortgage and checking accounts - it will work in the same way but less efficient and a lot more work to do all the transfers...