I'm thinking of something like "hairpin" or "miniature": this hairpin/miniature piece of knowledge is only useful when someone is choking on a black, titanium ball-pen while sitting on their head.
Perhaps you could try minute or minutiae to be more precise.
noun the small, precise, or trivial details of something.
If you're looking for an adjective to describe something really small and detailed, well you could use minute or intricate.
attentive to or concerned with even the smallest details:
very complicated or detailed
Is itty-bitty (aka itsy-bitsy) the figurative expression you're looking for?
itty-bitty: extremely small, tiny
Particularly relating to knowledge or information, you could use nugget:
anything small but of great value or significance: nuggets of wisdom.