RFID (Radio Frequency Identification) is a technology for object identification and tracking through radio frequency, which consists of RFID tags and readers. A tag in a passive RFID system is called a passive tag, which does not require an internal battery or power source and relies on radio energy sent by the reader to operate.

The passive sensing mechanism involves the following steps:

1. Transmission: The reader sends radio signals via the antenna. This signal contains information about identification and communication.

2. Induction: The antenna in the passive tag receives the electromagnetic signal emitted by the reader.

3. Power Harvesting: Passive tags use received radio energy to activate internal circuits.

4. Identification: After the passive tag receives enough energy, it responds by sending its own unique identification number (ID). The unique ID of the tag is usually pre-programmed by the manufacturer.

5. Communication: After the tag sends the ID, the reader receives it as input. Thereafter, the reader and tag can communicate within a certain distance.

The key to passive RFID tag systems is that the reader supplies enough energy to activate the tags and communicate via radio signals. This mechanism allows passive RFID tags to be sensed and read without contact, without direct contact or battery replacement of the tags. This characteristic makes RFID technology widely used in logistics, inventory management, identity verification and many other applications.