Why not just read priciples of Helicopter flight. Great book which goes into quite a bit of detail. Excellent book for instructors and commercial students. Maybe a little to heavy for primary guys.
I used to explain it in terms of enery management to students to keep it simple. I do agree with Eurochopper and Mike Smith in relation to the airflow. However I feel it is easier to explain to new students in energy terms.
i.e. ; Your coming down in your glide with lots of airspeed and altitude stored. When you get to the flare you start to bleed off that airspeed (energy), and that rate of decent (energy) this energy has to go somewhere it doesn't just disapear. Therefore it goes towards the RPM (causing the RPM rise as it now has more energy than it had before). Blade coning really isn't that large a factor in RPM rise.
Now sure this isn't a scientific explaination with lots of vector and airflow diagrams. However in layman terms it is why the RPM rises in a flare and it is far easier to grasp when you are actually doing the auto. I have never flared and thought of a vector diagram, however maybe I'm abnormal. I'm always thinking about how much energy I have left to use to keep me unbroken.