The button is used to aid the pilot in holding a precise hover, it is not intended to allow hands off hover.
Basically, when HOV is selected, a set of acceleration sensors is linked into the sas, so that a drift produces a slight counter cyclic, automatically. It is called hover augmentation on some helos.
If you need to hold a precise hover in some bumpy conditions, it is a definate help.
The SAR mode hover uses this hover acceleration feedback, and also adds the doppler/inertial, a much more powerful sensor.