all it does is set which channel the two drives are using on the cable so the signals don't get mixed up on that same cable. channel 0 is the master while channel 1 is the slave. it has nothing to do with boot priority or one drive controlling the other.
while that matters to the tech world, its easier to just have the jumper in the "cable select" setting. in fact, most IDE devices nowadays are shipped with the jumper already set in the "cable select" setting to make the drives easier to install.
See the struggle of the faithless lot as they negate their time
How low to sink to the depths of their frame of mind