"zero voltage drop diode"...Really?
He's talking about biasing a diode with a voltage source. 







simulate this circuit &ndash; Schematic created using CircuitLab

I've taken the liberty of adding C2 since the output will have some offset, and left out his "tweaks". The threshold will depend on the ratio of currents between the two diodes. The current through D2 will obviously be less so there will be a non-zero threshold, but much less than without bias.