Final answer:
Pinching your nose compresses the nares and affects the olfactory epithelium, which contains olfactory receptors that detect odors.
Step-by-step explanation:
When you pinch your nose to impede a smell, you are compressing the nares or nostrils, which are the external part of the nose. Just behind these is the internal nasal cavity, which includes the olfactory epithelium. This epithelium is key to our sense of smell and contains specialized sensory cells, called olfactory receptors, that detect odor molecules. These receptors and their associated nerves line the top of the nasal passages and transmit signals to the brain when they sense odors. By pinching your nose, you prevent air from flowing over these receptors, thereby diminishing your ability to detect odors.