I shall probably be shot down in flames by those that know a lot more about aerodynamics than me, but I'm pretty certain the nose down is simply "flap forward" ie reduced pitch from lowering the collective - - precessed through 90 degrees - - nose down
You mention throttle chops which is another area of debate. Personally, I think they were a good thing. Where I did my primary training they were routine right from when you first started doing autos (ie pre PPL). I also know there have been accidents / fatalities caused by them. As always, a double edged sword.
Is the added realism worth the risk? To quote from that first passage again "Bob also makes the point that messing up the autorotative landing flare is usually not fatal, but messing up the autorotative entry almost always is"
Surely throttle chops have more value than full-down autos? Admitedly, they require a lot of locking of limbs on the behalf of the instructor to make sure it doesn't all go pearshaped.