The rear element is λ/2 from the rear plate, so any reflected waves from the rear plate are in phase (effectively increasing the signal), the front element is λ/2 from the rear element so you have maximum deflection between the two elements (hence the diff amp). The 'beauty' of this is it is tuned to in-band signals (although some harmonics would also be amplified) so S/N should be significantly increased. The mechanical positioning of the elements (dependent upon frequency) gives this advantage.