The door is pretty well made and does exactly what you think it will. It allows you to plug a cable into a USB port so you can use the battery to power your Go Pro. I have a Hero 9 and with a powerful adapter and the included battery you can get 4-6 hours of recording. Please note that your GoPro is then no longer waterproof. With the cable connected it should handle light splashes, but I wouldn't trust it in the rain. When the cable isn't connected, the door stays closed, so it's probably fine in light rain. Anything water related should have an original GoPro door, not this one.
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