Open your HELOC agreement to the page with the word "index" on it. The rate is built from two pieces. The index is almost always the prime rate as published in the Wall Street Journal, which moves whenever the Federal Reserve moves its target rate, since prime sits three points above the top of that range. The margin is a number the lender added when they approved you, based on your credit score, how much equity was left after the line of credit, and whether the account came with an introductory rate. Prime plus 0.5%. Prime plus 1%. Prime minus 0.25% for the best files.
The margin is fixed for the life of the line of credit. The index is not. So when the Fed cuts by a quarter point, your rate falls by a quarter point at the next billing cycle, and when the Fed raises, same thing in the other direction. At the end of the day the only piece you ever controlled was the margin, at the application, and that's the number to shop. A half point of margin on a $60,000 balance is $300 a year, every year the balance is there.
Then there are the guardrails. Most agreements have a floor, a rate below which the line of credit won't go no matter what prime does, and the floor is sometimes set at the rate you started with, which means a rate cut can leave your bill unchanged. Most also carry a lifetime ceiling, commonly in the high teens, and some cap how far the rate can move in a single year. Find all three before you count on a rate cut, or assume a rate rise is limited.
Introductory rates are a margin trick. The lender waives part of the margin for six or twelve months, then the full margin kicks in. Fine if you know it's coming. Not fine if you sized your chunk payback around the teaser. Read the rate that applies in month thirteen and plan on that one.
Relationship discounts are the one lever left after closing. Many lenders knock a quarter point off the margin for autopay from a checking account at the same bank, and some will re-price the margin if your credit score has moved up a lot since the line of credit was opened. That's a phone call, and the worst answer is no.
The stress test on this site assumes rates move against you, and the margin and the caps are the inputs for it. Pull the agreement, write down the index, the margin, the floor, and the ceiling on one line, and keep that page with your monthly review. If the margin is above one, quote it to two other lenders and see who beats it.