I have a HELOC that I *can* use to purchase a property if my cash reserves are down at the moment. Have used it once in that manner. Just make sure you have fallback plans in case things go wrong -- like it won't rent or you lose your job, so don't use it to build a house of cards.
^This. You theoretically can take out a loan against your house to purchase investments or buy another rental property, but you must make sure that you can cover the payments of both your mortgage and the new loan with your current income, and you need to be reasonably sure that your income is stable. If this house is your primary residence, you run the risk of losing your investment property AND your home if you plan to use only rent from the new place to cover repayments of your HELOC. That puts you in a very risky situation.
How we have used a HELOC is to use a HELOC on a paid-off investment property (paid off from renting it out for several years) to purchase another investment property. The rent from the paid off property is added to the rent from the new property to pay off the new property more quickly.
Another way it could work is, if your current property is in a high demand area, you could move into the new property (if it’s not renting out), and rent out your current place. If you live in a city with low vacancy (for example, our condo is in Toronto where the vacancy rate is ~1-2%), then taking a HELOC to invest further is not a bad idea.