To limit me to the question: I first learned to place my hands on uke's shoulders and 'pull' uke down by stepping to my left rear. Nowadays I do it with a palm heel strike. The objective is to make uke's knees go forward which prevents uke to walk backwards when you step backwards. Walking backwards and turning towards you is what's going to happen if you just put your hands on his shoulders unless uke is very co-operative.
And to not limit me to the question: Uke should be unbalanced before you apply your hands to uke's shoulders.
And don't ask me why it is an atemi waza, I don't know, apart from the usual explanations.
Just one remark: all the atemi are done after the initial kuzushi.