Intersting how they did that.

I have a '93 Trek MTB that has toptube and seat tube front derailleur cable routing but uses a standard low-normal (LX) front derailleur that works in the standard fashion.

There is a cable housing stop bolted to the the fd's normal cable anchor point. The housing enters from above and stops on it but the cable passed through and is anchored to a screw in the seat tube just above the bb shell. Operating the shift lever pushes down on the housing and pivots the fd's operating arm down just as if the cable was pulling from below.