You'd probably agree there is almost always more than one logical / rational next step to consider, right? Decision making in chess is about weighing different probabilities, long-term outcomes etc. How do you decide that you've reached the most sound conclusion? How do you reach a conclusion at all?

The unconscious part of that decision-making process lies in the moment just before you determine you've made your most sound decision. While you were circling through different options, at some point your unconscious brain decided it knew enough, and converged on a decision. This choice has been shown to take place fractions before you become consciously aware of the choice. If a neuroscientist were reading out your brain signaling while you were playing chess, they would know you've reached a decision right before you yourself would know you'd reached a decision.