The iButton Serial Number is a technology based on the one-wire communication protocol, and a DS1990A chip packed in a robust stainless steel casing. This button-shaped device has two contacts - the lid and the base. These contacts carry the necessary connections down to a sensitive silicon chip, embedded inside the metal button.
When the iButton touches the reader probe on the iButton Click Board™, it establishes the communication with the host MCU, via the one-wire interface. The communication is almost instant, so it is enough to press the iButton lightly to the probe contacts.