First things first:

In a typical tandem configuration the tail produces negative lift to counteract the moments produced by the wing. This is essentially what gives the plane stability (and yes - for you engineers this is GREATLY simplified).

The lift produced by an airfoil is a function of the geometry (CL), the density and airspeed (q), and angle of attack (alpha). In a dive, the airspeed increases. Thus for the Pfaltz, the (negative) lift of the tail increases which pitches the nose up.

This is a simplified analysis, but I hope it gives you an idea of what's involved.