"add the interest for the next 5 payments and divide that by how much you paid on the principal during that time"
Let's see - on a $200K 6% loan, the first 5 months is $4869. Principal paydown is $1127. I get 4.32 or 432%. But this is nonsense, you divide the interest over the mortgage balance, and get 6%. You only get those crazy numbers by dividing meaningless ratios.
The fact that early on in a mortgage most of the payment goes to interest is a simple fact of the the 30 year nature of amortizing. You are in control, just add extra principal to the payment, if you wish.
This idea sounds like the Money Merge Account peddled by UFirst. It's a scam if ever there was one. I wrote about it extensively on my site and have links to others as well. Once you get to this page, the first link is for a free spreadsheet to download, it beats MMA every time and shows how prepaying works, no smoke, no mirrors. The second link is a 65 page PDF that compiles nearly all my writing on this topic as I was one of the finance bloggers doing what I could to expose this scam. I admit it became a crusade, I went as far as buying key word ads on google to attract the search for "money merge account" only to help those looking to buy it find the truth. In the end, I spent a few hundred dollars but saved every visitor the $3500 loss of this program.
No agent who dialoged with me in public could answer my questions in full, as they fell back on "you need to believe in it." I have no issue with faith-based religion, it actually stands to reason, but mortgages are numbers and there's order to them. If you want my $3500, you should know how your system works. Not one does, or they would know it was a scam.
Nassim Taleb, author of "The Black Swan" offered up a wonderful quote, "if you see fraud, and do not say 'fraud,' you are a fraud."
The site you link to isn't selling a product, but a fraudulent idea. What's most disturbing to me is that the math to disprove his assertion is not complex, not beyond grade school arithmetic.
Update 2015 - The linked "rule of thumb" is still there. Still wrong of course. Another scam selling software to do this is now promoted by a spin off of UFirst, called Worth Unlimited. Same scam, new name.