how does the preading rate vary from north to south along the pacific rise? why?

Question

the earth's surface relative to another can be represented by a rotation of that portion about a particular point; a pole of rotation. The position of each thumback is the approximate location of a pole of rotation of one plate with respect to another.

Answer 1

I guess you are reading the description of some kind of science project that uses thumbtacks on a globe(?) to show poles of rotation (Euler poles).
Here's a website that allows you to calculate the spreading rate at various latitudes along the Pacific Rise and other spreading ridges--and also gives you the locations of the Euler poles for these segments:
http://ofgs.ori.u-tokyo.ac.jp/~okino/calc.html
Select #8 -east pacific rise, and put in some latitudes and you will see how the spreading rate varies from -30 or so to the equator. The Euler pole for the Pacific-Nazca ridge is Latitude 55.6 [deg.]Longitude : -90.1 [deg.] Note, the Euler pole is south of the segment.
I don't know whether you are supposed to include an analysis of the Cocos-Pacific (#6) and Pacific-Antarctic (#10) segments.--but you can get spread rates and Euler pole locations at this same site.

Basic information to help you picture the influence of the Euler pole on spreading rates --in general, the farther from the Euler pole, the faster the spread rate.
http://www.earth.northwestern.edu/people/seth/Demos/BRICK/brick.html