HELOC repayment on adjustable interest rate?

[Ben Kirchner]

I'm looking into taking out a HELOC for a future investment. I understand the HELOC fixed rate repayment (just like a mortgage). What I'm not sure I understand is the HELOC repayment process on a variable interest rate. To my understanding, a variable interest rate would only have an interest rate on the balance of the HELOC, and therefore would change throughout the loan, as I paid down the HELOC.

Example:

I borrow $80,000 on my HELOC at a 6% interest rate. I want to pay down $20,000 off the principal in the first year.

Year 1 - I pay it down to $60,000 principal.  I'm then paying 6% interest on $60,000, right?

Year 2 - Paying on $40k?

Year 3 - Paying on 20k?

In this example, my monthly payments would get lower and lower, instead of being the same throughout the life of the loan.  This of course has me thinking if I owe around $80k on a mortgage on a rental property , rather than paying 5.5% interest on a fixed 30 year term, what if I paid a variable interest rate on just the $80k.  I know I've seen some talk of this on other websites, and wasn't sure if it was a legitimate strategy, or if I was missing something.  Anyone who could provide more insight, I would appreciate it.