Almost never on a door that is actually locked. Locksmith guides say the trick only ever worked on a simple spring latch, and that a deadlocked door, a night latch that deadlocks or a uPVC multipoint lock all stop it working. What it usually does is ruin the card and damage the door seal.
The trick depends on a simple sloped latch, the kind that clicks shut when you pull the door to. A latch like that holds the door closed but does not lock it. Once a wooden door is deadlocked with the key, or a uPVC door has been locked by lifting the handle and turning the key, bolts or hooks are holding it and a card has nothing it can move.
Night latches can often be deadlocked as well, and a deadlocked latch cannot be pushed back. That is why locksmith guides say nearly every locked British front door is safe from the card. If you try anyway, the usual results are a bent or snapped card and a torn weather seal, and you are still outside.
If a card could open your door, that is worth knowing for another reason: anyone else could do the same. A door left on the latch is closed, not locked, and Admiral warns that if your doors are not locked and you need to make a theft claim on your home insurance, you probably will not be covered. Locking with the key each time you go out closes that gap.
If you are locked out right now, a spare key or a locksmith is a better use of the next ten minutes. The Master Locksmiths Association says a locksmith has a number of techniques for opening a night latch without drilling it or damaging it, so a door that was only pulled shut is often one of the simpler lockouts to deal with.
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