What is ‘Hay’ Assessment

Hi, due to a re-writing of my job description, i am currently some kind of IT Manager,,,/Net Admin/Staff trainer/Phone Guy/Graphic Designer/dog walker etc, (i lied about the dog walking but i do act as a help desk) (it’s similar:).
My line manager wants to subject me to something called “Hay” assessment? aptitude/suitability probing, can’t pay me more ’till they know that i can really do these things i do!

Any help e.g. what to expect, is it painless? a dummies guide or sure fire answer sheet might help.

Over to you my Revered Republic, thanks.