Yup yup - just like pulling it - take the hairs from underneath and do as you would if you were pulling - therefore getting the longest. Make sure you cut right at the top though.
Ive just got my new one and what a difference sharp blades make. If his manes thick definately use it as if you are pulling, back comb and cut otherwise you end up with a bog brush effect. It seems to shorten anyway as pulling properly would if used this way.