The mags go straight to the ring of the Stereo Switched jack, instead of going through the Acousti-Phonic (true bypass).  There is no blended (mono) mode available in this configuration.  When you insert a stereo 1/4" plug you get mags on the Ring and acoustic on the Tip.  When you insert a mono 1/4" plug, you get acoustic on the Tip only.  The Acousti-Phonic is held in mono mode by grounding the ring contact permanently, but the signal going into it is acoustic, so that's all that appears on the Tip.