All objects with a temperature above absolute zero (0 Kelvin / -273.15 °C) emit heat in the form of infrared radiation. The warmer and/or larger the object is, the more radiation it emits.
A PIR sensor (Passive Infra Red sensor) is designed to detect this infrared radiation.

A PIR sensor consists of two sensors positioned next to each other in a metal housing behind an IR permeable window. These sensors are made of IR sensitive material, pyroelectric sensors. These sensors detect IR radiation. A PIR sensor measures the differences in detected radiation between these two sensors.
When everything is at rest, both sensors will receive the same amount of IR radiation, this is the ambient radiation. The sensors and thus the smart-alarm will adjust to this.

If a warm object, e.g. a person or an animal passes by the PIR sensor, it will first enter the "field of view" of the first sensor. This means a difference in IR radiation between the two sensors in the PIR sensor. On this difference the PIR sensor reacts and thus the smart-alarm. When the warm object passes the PIR sensor, the first sensor will return to the normal or idle position and the second sensor will respond to the warm object.

Both sensors in the PIR sensor are in switched in the opposite direction. The first sensor will give a positive signal for a detection and the second sensor a negative signal.
Both sensors are switched in the opposite direction so that in case of a sudden change of ambient radiation, this is detected by both sensors and the signal from both sensors will cancel each other out (sum of both signals) and this will not cause an alarm.

A PIR sensor and the smart-alarm will therefore only react when an object passes by. In practice, the smart-alarm will also react to an object approaching from the front, but this will only be a few meters away.