If you want it shorter without pulling I would say to just cut it then take the scissors and go across cutting up vertically a little like a hair dresser, if that makes any sense haha. If its just thick I would use thinning scissors, but my horse and I show HUS and I button braid him so I just cut his mane. (:
I flip the mane on the opposite side, cut it straight across with sissors, flip it back over, and voila!!
No need to pull or even out after that, unless the mane is just really flat out insane. I don't know how or why it works, but it does. It's what I've done for a while, and it looks the same as all the horses with pulled manes.