Most of us know, or think we know, which way we need to adjust it. There's no computers on board. It's in the driver's hands and you've got to make the right calls to try and do the right thing. Through years of experience and thousands of laps, drivers can decipher what changes need to be made based solely on senses. Knowing what he knows, knowing what he feels in the car, he can dictate what you need to do.
Good communication is critical.
Before radios were common in the sport, teams relied on hand signals to communicate. If you had a loose condition, you motion to the bottom side of the door. You could live without the radios. Drivers rely on two-way radios to stay in communication, discussing track conditions, weather and potential adjustments. The decisionmaking process is anything but an exact science.