..if you are not used to managing the AP and you lower the lever to start your descent without disengaging the ALT, the AP will bring the nose up to try and maintain height and reduce your airspeed.
Exactly. Or if you're in a descent at reduced power with ALT PRE armed, when ALT is captured and power left unchanged, pitch is going to ever so slowly pitch up to maintain altitude, and then all of a sudden when you're on the backside of the power curve you will get a sudden pitch up and things will get very exciting very quickly. As Hevilift found out in Indonesia in their S76 they pancaked into the swamp.