expansion valve testing?

educate me on the sense bulb. Its the bulb at the end of the capillary tube but at what temp does the thing do something? Hot or cold to open the valve? Im not versed in A/C so Im assuming the valve is shut until the bulb changed temp and then the valve opens to start letting the refrigerant into the condensor? Im guessing if the valve doesnt open, no cooling (ie. a ice or debris clog would render it inoperative?) It pisses me off more than my son who drives it all the time. I hope the thing is not behind a 2 hour dash removal....