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How NFC phone blockers work, in plain English

4 minute readBy the moss team

An NFC phone blocker looks like magic the first time: you touch your phone to a small object and half your apps go grey. There is no battery in the object, no Bluetooth and no Wi-Fi. Here is what is going on.

The puck is a tag, and the phone does the work

NFC stands for near field communication. It is the same technology behind contactless cards and transit passes. It runs at 13.56 MHz and works over a very short distance: the NFC Forum describes a typical range of up to 2 centimetres.

Inside the puck is a passive NFC tag: a tiny chip and a coil of wire. It has no power of its own. When your iPhone comes close, the phone's NFC reader creates a small field that powers the chip for a moment, and the chip answers with its data. That is why a puck never needs charging and should last for years.

Each tag also carries a hardware identifier. Apple's developer documentation describes it as "the unique hardware identifier of the tag". (Apple Core NFC) Whether an app uses that identifier matters a great deal, as we will see.

The lock itself is Apple's

The puck does not block anything. It is a trigger. The blocking is done by your iPhone, using a set of tools Apple provides to developers, usually called the Screen Time API.

The app first asks your permission to manage Screen Time on your device. Then it shows you Apple's own app picker. Apple's documentation notes that this picker lets you choose apps and websites "without revealing their choices to the app". (Apple FamilyControls) The blocker receives anonymous tokens, not names. It can say "shield these tokens", and iOS does the rest. (Apple ManagedSettings)

So the sequence when you tap is:

  1. The phone reads the tag.
  2. The app checks it is a tag it accepts.
  3. The app tells iOS to shield the apps you picked.
  4. iOS greys them out and shows a shield screen if you try to open one.

Tapping again reverses step 3.

Why a physical tag at all?

Because the tap could be a button. Many blockers are exactly that: an app with a "start" and "stop" button. The trouble is that the stop button is on the same phone, in the same hand, at the moment you want to give in.

A tag moves the off switch out of the phone and into the world. If the puck is by the front door and you are on the sofa, unlocking costs you a walk, and that small cost is often enough. Economists call this kind of arrangement a commitment device: something you set up in a calm moment to make it easier to stick to your own plan later. (Bryan, Karlan and Nelson, 2010)

It also explains the main piece of advice for any of these products: keep the tag somewhere inconvenient. A tag in your pocket is a button.

Which tags unlock which phones

This is where products differ.

  • Brick accepts any Brick. Its support site says "One phone can work with any Brick device in the world." (Brick support) Convenient for sharing, and it means any other Brick is a key to your phone.
  • Foqos, a free open source app, lets you bind a profile to one specific tag or QR code, so no other tag stops it. (Foqos)
  • moss ties its stone and sprigs to your household. Only your own ones unlock your household's phones.

Can you get around it?

Yes, and anyone who tells you otherwise is selling something.

Because the lock is Apple's Screen Time, the permission you granted can be withdrawn. Go to Settings, find the app, switch off its Screen Time access, and the shields drop. This has been true of every third-party blocker for years. A long-running thread on Apple's developer forums describes it, and Opal, a software blocker, lists it among the ways around its own product. (Opal help)

There are two defences.

A Screen Time passcode you do not know. According to Opal's help centre, from iOS 26.4 a Screen Time passcode is required to switch off an app's Screen Time access or delete it, and Face ID cannot stand in for it. Ask someone you trust to set the four digits. Brick suggests a passcode as well. (Brick support)

Strict modes. Most blockers, including Brick, Bloom and moss, can stop you deleting the app while a lock is on.

moss adds a social one: in a household, a person who switches Screen Time off shows as "Not protected" to the others. It does not stop you. It does mean you cannot do it quietly.

What happens in an emergency

Since the lock is real, each product needs an answer for the day your phone is locked and the tag is across town.

  • Brick: 5 Emergency Unbricks, reset on request.
  • Bloom: 3 emergency exits, plus one more each month.
  • Blok: emergency unlocks sold at $4.99 each.
  • Foqos: none.
  • moss: 3 a month, refilled automatically.

In every case, phone calls keep working and so does anything you did not choose to lock.

The short version

The puck is a passive tag with no battery. Your iPhone reads it from a couple of centimetres away and then uses Apple's own Screen Time tools to shield the apps you chose. The tag is there to put distance between you and the off switch. It can be bypassed through Settings unless you set a Screen Time passcode, and it is still far harder to slip past than a limit you can dismiss with one tap.