DO YOU USE CASH FLOW TO PAY BACK HELOC

Hi! I'm a new investor and i really want to scale up my portfolio by using some of the equity thats in my rental to do so. I've been advised on bigger pockets to use a HELOC as opposed to a cash out refinance (if i should cash out refinance when is it best?). My big question is, if i find a deal that's cash flows well and my interest is pretty good on the HELOC say 3-5% (hopefully fixed if i can find those lenders) is it smart to use the HELOC for down payment and small start up rehab, then pay it back with cash flow?? Or could i find myself under water with this strategy. What do investors usually do in this situation to scale using HELOC and how do they pay it back?