@drwho in a deadlatch setup there's a little latch that sits at the "top" of the regular one. If a door is correctly installed, then that little latch is depressed by the plate that the larger one clicks into. That stops the big one from retracting while locked.
*But* (way too) often the plate is too big or misaligned, and that lets the little latch extend, which makes the door susceptible to the latch tool.
Or, for most interior door latches, there is no deadlatch, and so the latch can be depressed manually with no issues.
One thing that can make it harder to execute the attack is a plate or moulding that covers the gap between the door and the frame. Angled or flexible tools can still work though.
@deviantollam