hi i just recently got a timing gun off my friend and i was wondering how it works theres a dial in the back to set the degrees like for instance if i want to set it to 17 degrees and i press the trigger how will i know if its at 17? does the light change or something? please let me know and while youre adjusting the distributor it is ok to move it while the engine is on while youre doing this right? im new to advancing timing any help would be appreciated thanks