This could get complicated if we aren't careful. It's all about vectors!

Click on the link and try and follow that.

Basically when you are in auto you have a rate of descent. This creates a 'rate of descent flow' which is the air coming up through the rotor disk. The rotor is also spinning so you have a rotational flow too (like across a wing on a fixed wing aircraft). The two vectors combine to produce a single vector that approaches the blade from below (as in the diagram).

Lift is always perpendicular to the air approaching the aerofoil (irrespective of the blade angle). At certain points along the blade where the correct conditions exist between the 'rate of descent flow' and the 'rotational airflow' the resulting lift vector is angled slightly forwards. This now has a horizontal component of lift pulling the blade round. This is the autorotative section of the blade and what stops the blade slowing down.

As can be seen from the diagram you do not need negative pitch for this to occur.

I hope someone else can make sense of my ramblings!