We have a lot of tips and tricks to help you shop Amazon Prime Day. After all, we have a whole Institute of experts dedicated to finding the best products and a slew of shopping pros looking out for the best sales. But one of our favorite shopping hacks has, not surprisingly, come from Reddit.

On one thread called “A tip for Prime Day—Be Informed and don't get taken,” users were quick to comment with their favorite shopping tips. One that caused the most interest was using Auto Buy, an under-the-radar Amazon feature that can cut your shopping time in half.

Think of Auto Buy as your personal, virtual shopping concierge: Prime members can ask Amazon to track an item they have their eye on, and automatically buy it when it hits a certain price. For example, you can ask Amazon to buy a specific cast-iron pan when it’s on sale for under $100 or a buzzy air fryer when it’s finally more than 20% off. Prime members simply click “Add to Auto Buy” on a product page and create a shopping rule. You can also have a reminder sent to you instead so you can decide if you still want the item.

Product listing for Apple AirPods Max 2 priced at $399 with auto-purchase options.

“I added several things to my cart during the past few weeks so I can easily monitor any price changes before and during Prime Day,” wrote a Reddit thread’s original poster. “So I know if [or] when to pull the trigger, and noticed something interesting.” “This is why I love the auto buy feature,” another person commented. “Set your price and walk away.”

Though Auto Buy is a clever hack for avid Amazon shoppers, it does have a few rules. For starters, this feature only works for items that are fulfilled by Amazon. You can have up to 200 active Auto Buy rules, but you can only have one request per item and Auto Buy one unit per item. Also, you can’t use promo codes or coupons on your Auto Buy request.

That said, if you’re OK with the fine print and want to make the most of Prime Day, Auto Buy is poised to become a game changer.