Since its my tertiary function it might work differently for me than for you INxJ types, but basically what happens to me is the sum of all my observations are subconsciously stored and analyzed and without purposefully using logic or reasoning I come to a conclusion using all the bits of information I've collected. It's almost like I don't have control over it, it just does what it does. What I can then do is think back through my day or through the relevant information I've collected to see how I got there, but I have to get there unconsciously first before I work my way back and figure out why. Note: this isn't the only or even main way I come to conclusions, but it is the way I come to my more profound and life-effecting ones.
Can any other ISTPs relate to this?