It is just an audio spectrum plot - the program I'm using plots the frequencies in the recording, and you can see the spike representing the fundamental rpm, as well as the harmonics at integer multiples above that. You can select a section of the recording that you want to look at - which can be long, such as the 20sec section I looked at here. You move the cursor to where you want and read the frequency off the display, then just multiply the frequency by 60 to get rpm. It was documented here:
http://www.arboristsite.com/community/threads/measuring-rpm-from-video.281228/
It has been controversial among some who are not familiar with it, although it is a routine technique used in all sorts of things. Some people think it's something I invented, which is laughable.
I used a 2sec window centered at those points:
1:43 13020rpm
1:50 10860rpm