Its a completely valid strategy to use equity to service debt. In fact you'll find people use this ("debt recycling") to replace a non-tax deductible home loan (owner occupier) with tax deductible debt.
For example: you own a $1m property with a $300k mortgage thanks to property growth. Draw down $500k to buy another investment property. Your LVR on your home is now 80%.
With the $500k, put a $250k deposit down on a $1m property (or portfolio of properties) with a rental yield of 5% and an LVR of 75%. You've borrowed an additional $750k (all at 9% for argument's sake) for a total debt of $1.25m. (excluding your $300k home loan).
It takes $112.5k per year to service that debt. Your property/portfolio yieelds $50k per year for a shortfall of $62.5k. Even at a 40% tax bracket, thats a deduction worth at least $25k per year to you.
You take that $50k and use it to pay off your home loan. The remaining $250k from your original drawdown is used to service the investment debt. You can balance these numbers as a function of rental yield, interest rates, etc. The idea is to get the right LVRs such that by the time the equity used to service the debt is gone you've repaid your owner occupied loan and then you can use the money you were using for that to service the investment loans. Hopefully by then (in ~5 years) the rental yield is also higher.
What are you left with? A $500k loan on your home thats fully tax deductible (hence the term "debt recycling").
Of course you need to consider transaction fees and its still important to buy the right property.
Assuming property growth of 10% then after 5 years your $1m investment has appreciated about $600k, your home loan is tax deductible and the rental growth will have (hopefully) increased to the point where it covers or almost covers the $750k loan.
This strategy works best is a rapidly rising market because its highly leveraged.
Anyway, don't take this as advice.