Most likely, the mosfets are simply used as clipping diodes, and the switch lets you choose between the mosfet clippers and the regular SS diode setup.
I'm interested in this my self, This is what Mike emailed to me when I was asking him about it!
A Mosefet Modded FD2 is Great...OD mode is way more open in FM, and a HUGE difference between FM and Vintage. Great distorted tones with more detail without any fizz...and the Boost channel is very dumble!
Nice, that should give it some more variety and compete with the smoother Mosfets like the Blackstone/Mosferatu etc....and wasnt the OCD mosfet clippers? maybe integrating some of t he OCD into the FD2 would be nice....