@Mark Buskuhl makes a good point on the cost of credit, a HELOC is very in-expensive to secure the credit upfront versus what I learned about the BRRRR method is two things 1) typically can't go over a 75% LTV on a cash out refinance and the origination fees are infinitely steeper than a HELOC and can end up in the $1000s. With that said you need to make a significant improvement on the property for the BRRRR otherwise the cost of getting that equity back out can put it well over 10% (and you are just taking it out to invest in a 7 cap), which means its cheaper to not refinance in the end and leave the equity in the rental.
You can typically lock a rate with a HELOC after you a balance on it you can "freeze" that balance at a rate. So if you knew you had a $50k HELOC, used a lot of it, you could freeze $25k of it at todays rate to avoid the time risk of the rate going up.
Message me if you want to chat more, I have done a BRRRRR, HELOC, Cash out refinance, traditional refinance all in the last 18 months.