Most effects circuits have a pull down resistor connected between input and ground to reduce the possibility of the footswitch popping when activated: It's usually 1M or higher to minimise potential "loading", i.e. keeping the input impedance as high as possible to reduce rolling off too much of the low end.
However, the interaction of a guitar played direct into a fuzz circuit (e.g. quality of response to rolling back the guitar's volume pot) is often dependent on the input impedance. As I understand it, the fuzzface is particularly good at cleaning up due to the relatively low input impedance the two transistor circuit yields compared to the output impedance of a guitar (especially single coils with a low resistance volume pot). The TB mk2 is effectively a rangemaster-style single transistor boost stage linked direct into a fuzzface circuit: It doesn't tend to clean up as well as the FF and this may be due to the fact the guitar pickup signal passes through the boost first and is not directly linked to the FF input. I think the loading pot aims to improve the response of the fuzz to volume pot manipulation - Lower resistance to ground will probably allow better clean up at the expense of some gain and low end. You are effectively varying the value of a pull down resistor from quite low (102k2) to extremely low (2k2)