How to plumb 3 port diverter for shower, handheld or both

I've read that a 3 port diverter can be plumbed to allow a fixed shower head and a handheld to operate separately or at the same time, but I'm having difficulty understanding how. I figured that each fixture would be supplied by a separate port and then the 3rd port would supply both, but it seems that would effectively connect the supply pipes in a way that will result in water always coming out of both. What am I missing?

If the diverter has that capability, and not all do, then the internal mechanism is what joins them for simultaneous operation.