How it works by thetripp from Reddit:
Water in tissue tends to move around via diffusion, and this diffusion can very slightly change the local magnetic properties of tissue. To measure it, you send out two pulses separated by a small time window, and measure the decrease in signal caused by diffusion. You do this in a way that allows you to determine which direction that diffusion preferentially occurs in. Then you assemble these signals for nearby areas, and what you see is that diffusion tends to track along vascular/fibrous structures.
Also keep in mind that MRI is fucking complicated and hardly anyone understands it well. I studied MRI briefly at the graduate level, and I do research now on advanced methods in computed tomography (CT). MRI is still largely a mystery to me in terms of how it actually works.