Yasgur’s Homeopathic Dictionary describes a Keynote symptom as a,
…leading symptom, one which is so apparent, so clear, that it suggests
a small group of remedies or even a single remedy.
For example, pain
in the right shoulder blade points to Chelidonium. Keynotes are
actually peculiar symptoms which have taken on a highly characteristic
flavour and tend to point almost directly to a remedy, yet if keynotes
are taken as final and the generals do not confirm them, failures often
result. Many of the great homoeopathic prescribers (Lippe, Allen,
Boger) were very successful keynote prescribers, but you must realize
that they had keen ‘totality perceptions’ and thus would not allow false positives to sway them.

Anyone who knows the basis of homoeopathy could try to learn In-depth some keynotes and start practising this. There for sure are homoeopaths who will not agree with me and will wish to hold everything “under control” [in their own hands]

Another 2 examples;

Thuja occidentalis : Sweat; only on uncovered parts.

Sulphur :
Standing is the worse position for sulphur patient, they cannot stand, every standing position is uncomfortable.