Most sensors have two internal slots. When you walk past, you enter the first slot’s view and then the second, creating a "differential" signal that the sensor recognizes as motion.
Proper placement is the difference between a reliable system and a headache of false alarms. How to Wire a PIR Sensor: A Simple DIY Guide Pir Sensor Manual
Every living thing radiates heat in the form of infrared energy. A PIR sensor contains a pyroelectric material that generates a voltage when it detects this heat. Most sensors have two internal slots