This is a heuristic I use to check whether there is a correlation between two things. By considering the extreme ends of the spectrum I can think through whether they align the way I would expect them to if a correlation existed.
For example: I once wondered if the grind size of coffee had any impact on the flow rate of water through the coffee bed. So, I considered the extremes cases: if I put the biggest grind size possible, i.e. whole coffee beans, into my coffee machine, water would shoot through the coffee bed. On the other hand, if I put coffee grinds the size of sand into my coffee machine, the water would drip through at best. Thus, it is likely that the two are correlated.
Caveat: correlation does not imply causation and intuition can be wrong.