Sorry Mike, I didn't realize that you were asking about the formula.

It's fairly basic in what it is saying. 1/Frequency gives the period in time between cycles. So the time interval of the signals coming and going are being added together, then converted back to the frequency (divided by 2). Then the conversion back to rpm.

A plane doing around 170 will have a doppler shift of about 23%, throw in the unload on top of that, and you will have a pretty big number when the airplane is flying toward you.