pulling helps it to thin the mane and help it lay flat - but if your horse's mane is already thin - may I suggest "thinning" shears... they are scissors with teeth, so they don't get all the hair at once. You have to make a few passes over one section - but I find that you get a much more natural look than straight shears, which leave you with a very blunt end. My horses mane is very thick, so I pin the top half up, shear the underside a bit shorter, and then do the top half a bit longer. You can also look at "humane" pullers, which are basically a hybrid between a pulling comb and thinning shears.
Good luck! Your horse is beautiful!