I thought because it's located by the head it was a crank angle sensor. It allows rotation and i'm sure is to adjust something when it's rotated. But what? timing?

 

That is the Cam position sensor, or cps, not the crank position sensor ckps , which is the teeth on the front dampner. It's sole purpose is to tell which turn of the crank you are on for firing the spark. Allegedly, the engine will try one then the other case during startup and still start, albeit with longer cranking.

 

So why is it adjusted? I turned it counter clockwise a few degrees and the car would not start. Seems weird to me. My first Jag BTW

 

well, I GUESS you passed the point where it evaluated the correct crank turn for th cam position. If you wish to experiment more, then disconnect it and see if it cranks, but after more turns. I have not actually done that, but I will bet on the result. The spark timing is done by the CKPS.