Short answer
Smell training is a form of olfactory exercise in which someone regularly smells a fixed set of different scents and consciously concentrates on the perception. It is being researched and applied for various types of smell disorders and can support improvement in certain patients.
Where does the idea come from?
The olfactory system is plastic: experience and repeated exposure can influence certain aspects of odour processing. Studies on olfactory training therefore use systematic repetition instead of a single strong stimulus.
What does training look like in research?
Many protocols use several clearly different scent categories that are smelled daily or twice a day for several months. The exact scents and duration differ per study and clinical context.
The goal is not to sniff as hard as possible. Regularity and conscious attention are more important.
Does it work for everyone?
No. Reviews find positive effects on average for certain groups with smell loss, but outcomes vary. Cause, age, duration of the disorder and the method used can influence the results.
With post-viral smell disorders, chronic nasal problems or smell loss after trauma, the medical context is different. That is why a single universal online protocol is not suitable for everyone.
Smell training in healthy people
Even without a smell disorder, olfactory exercise can be useful for better identifying scents and learning to recognise differences. That is skills training rather than medical rehabilitation.
A perfumer, for example, trains vocabulary, memory and discrimination. That is not the same as treating anosmia.
Keep expectations realistic
Smell training is not a quick "reset" of the nose. When used medically, it is usually a process of weeks to months and not a few days.
First understand the cause
Anyone who clearly smells less should have it checked whether there is a treatable cause. Training can be part of it, but it is no substitute for examination of the nose, sinuses, medication or other relevant factors.