Blocking from behind

[Jersey Ref]

This year I have been hearing a lot of "how can the be a block in the back his head was in advance of K's player"
I read the rule and as far as I can tell if the intial contact is in the back no matter where the head is it is a block in the back. I have had officials tell this to me also. can some one help me I want to make sure I don't blow a call because of something I don't understand. Thnak you for everyones help