Baobab
When you bring the cyclic back, you are raising the nose and changing the angle the air meets the blades - this will give more lift (and drag) but the extra lift will make the blades try to fly up (cone up). As the blades cone up, their centres of gravity will move towards the rotorhub (only a small amount) enough to increase the RRPM because of the conservation of angular momentum (think of a spinning ice skater who brings her arms in to spin faster and out to spin slower).
The opposite is true when you move the cyclic forward (this can be very dangerous in a teetering head helicopter if you do it quickly as it can lead to mast bumping).
In summary - bringing the cyclic back increases RRPM in auto (and reduces speed) and moving it forward decreases RRPM (and increases speed). It is the rate of change of the airflow that determines how quickly the RRPM rises or falls.