The Big Fuzz is a muff circuit, which has been made to factor all of my favourite elements from the many various models. Further, some improvements have been made to improve user interaction. This includes increased bass, and improved bass clipping response, improved tone control functionality, and fine tuned top-end clipping response and smoothing.
The classic design varied greatly over the years, being made in various locations to different schematics and ever-changing component values. One large reason I suspect some models are still higher favoured now, is for their amount of mid-scoop. This is a famous love-hate element of the circuit. However, having this second tone knob adjustment is incredibly helpful.
There are two internal mod switches to independently change the gain stages. This goes from the Russian style lower gain, which is famously a bit smoother and proportionately warmer in the top-end, to the modern NYC high gain which is a bit more aggressive. Combing this with the mid knob means you can achieve tones alike to many releases.
There is also an internal mod switch to transform the circuit to work as a Supa Fuzz (Marshal, Sola Sound). This has a huge bottom-end wrapping, and is the preferred mode for bass players.