Why do you need to short the "reset pins", or press the "reset button" when you install a new battery?
When you install a new battery into a digital electronic device like a wristwatch, oftentimes the connection "stutters" as you fiddle with the new battery; connecting, then disconnecting, then reconnecting, repeatly; all very fast (millisecs). And when that happens, it sometimes messes up the digital electronics and it gets into a weird state.
(Technical explanation: Digital devices all depend on "clock pulses" -- alternating high and low voltages -- and erratic power that pulses as you install the battery looks like a valid clock to the system. And that is bad.)
So, fussy electronic devices (not just watches) often need a "warm reset"; that is, a "reset" done with consistent power. That's exactly like pushing the reset button on your computer, versus yanking out the power cord and plugging it back in.
That's why you MIGHT need to do a "reset" after installing a new battery.
How do you know if you need to do a "reset"? Well, if the device is working correctly, then no reset is needed!
markj99, November 2009