Caffeine, niacinamide, & aspirin. Why?

I apologize that I am not well studied in this subject. When I think of niacin and using brewers yeast as on oral supplement I remember the red skin heat flush that happens if you take enough of it. It sounds like the niacin/niacinamide is targeting the removal of bad fat/oils or something in the preceding threads.
People are asking about how long to keep a topical solution with niacin applied. I wonder if it be kept on until a red skin flush appears on the scalp ? Obviously this redness is blood flow and a great benefit for circulation, and of course it means that the solution has been absorbed at a quantifiable level. Not sure if it is useful for the goals of the discussed treatment, but I find that cinnamon oil gives and instant red skin flush to aid in circulation also.

So nobody knows how long any kind of topical needs to sit on skin for a decent absorption?