If you have a 6% mortgage your first step should be to refinance, no? While doing that, you can switch to a 15 year mortgage (often at a lower rate), thus 100% guaranteeing that you will pay off your 30 year mortgage in 15 years if that's your goal.

There's no magic in finance, it's just math. I'm too lazy to make a spreadsheet, but this seems like silly advice, a "trick" for people who don't understand compounding. For one, principal payments will increase as the mortgage amortizes, so thinking that an extra $270/mo will cut this mortgage in half is not correct.