I found the POH online that I believe this procedure is from, and it describes a 1970's Baron. I see no reference to this system in the late model Barons after the 1984 redesign. If I'm reading between the lines correctly, I think this is also an antiquated safety system, which requires the pilot to manually disable the autopilot and/or trim servos via another switch on the panel while simultaneously holding the disconnect switch, as the yokes equipped with this trim disconnect button do not have the now common two-stage AP/FD disconnect button. I imagine this is still somewhat common in 60's and 70's airplanes today, but I have yet to encounter one. You learn something new every day!