This comes up constantly, and getting it wrong in either direction leads to a bad experience. So, plainly:
What a passive collector does
When you feed on one side, the other side usually lets down too. That milk goes somewhere — typically into a nursing pad, your bra, or your shirt. A passive collector sits over that other side and catches it.
There is no suction. No motor. Nothing to assemble or switch on. It holds position inside your nursing bra and your body does what it was already doing.
What it does not do
It does not extract milk. If your body is not letting down on that side, a collector will sit there and collect nothing. That is not a fault; it is the entire design.
It does not build supply. Supply responds to demand — to milk being removed. A collector catches milk that was leaving anyway, so it does not send your body the signal that a pump does. If your goal is to increase supply, a collector is the wrong tool and you want a pump and a plan, ideally with a lactation consultant.
It is not hands-free storage. It is an open collector, not a sealed bottle. Lean forward far enough and it will spill. Empty it as it fills.
Will it cause oversupply or make one side leak more?
No — it doesn't change what your body is doing, it only catches what's already happening. Letdown on the non-feeding side is triggered by hormones responding to your baby nursing, not by anything the collector does. Wearing one doesn't signal your body to make more or less; it just means milk that would have soaked a pad ends up in a container instead. If you notice an actual change in how much you're leaking, that's your supply adjusting for reasons unrelated to the collector — normal in the early months as things settle.
So what is it actually for
Two things, mostly.
The first is arithmetic. If you leak an ounce a feed and you feed eight times a day, that is most of a bottle going into a nursing pad daily, for months. A collector turns some of that into stored milk without asking anything more of you.
The second is dryness. Plenty of people use one purely because it beats sitting in a wet nursing pad for the length of a feed, and treat any milk collected as a bonus.
Storing what you collect
Passively collected milk has been sitting at room temperature, against your body, for the length of a feed. Treat it the way you would treat any freshly expressed milk: transfer it to a bottle or storage bag and refrigerate or freeze it promptly. Follow your local milk storage guidance for how long it keeps — it varies by country, and your health service publishes the numbers you should be using.
If you have been told to follow specific handling instructions — because your baby was premature, is in hospital, or has particular needs — follow those instead of anything you read on a shop's blog, including this one.
Related reading: How much milk are you actually losing to leaking? · Hands-free pumping without leaks: what actually helps between sessions