Push-Push Amp Frequency Doubler
The Push-Push Amp allows for a fully active full wave rectifier. The BJT splitter gives roughly unity gain ac signals of opposing polarity, and the following transistor pair both rectifies and amplifies the combined signals.
This topology is notable for providing gain (and plenty of it) during rectification.
Regarding the mysterious trimmer in the Superfuzz
With a modest input signal and using a trim pot to balance the rectifier (see Superfuzz schematic), a very accurate frequency doubling of a sine wave can be produced. Although I have never come across a textbook entry with this circuit, I imagine Fumio Mieda learned it as a radio signal processing circuit. The early Superfuzz variations have the trim pots installed, later ones the trims are missing from the circuit board, and later pedals omit the trim entirely. The trim is good at low input levels with a steady sine wave, but becomes useless at high gain and isn't required to make a good fuzz circuit.
Having serviced several Superfuzz's with low gain due to problems in the various stages of the circuit, there is quite a wide variety of fuzz/octave up timbres that the circuit can produce as the internal line levels are adjusted. The normal sound of the pedal is extremely high gain. Imitations of the gated No Bias circuit and the Octavia's clean-ish octave up are obtainable by modding the Superfuzz.
Examples:
Honey Baby Crying
Psychedelic Machine
Kay Fuzz
Mica Fuzz Machine
Boss DB-5 Boss Driver
Boss FZ-2 Hyper Fuzz
etc.
(Most Japanese octave up fuzzes seem to be Superfuzz based)