That is the reason why the late Blake Stevens labeled it "The Trigger Group from Hell"....
The whole idea was to combine semi-auto and full-auto in a single trigger assembly without a separate selector switch. It was an early attempt at what we'd call today a progressive trigger, in which the length of pull determines the mode of fire. Short pull is semi, longer pull is full. In semi-auto the MG34's trigger's travel is mechanically blocked from going all way back; pulling the lower portion of the trigger for full-auto lifts the block so the assembly can go back further and hold down the disconnector.
In practice it was more complexity and trouble than it was worth. One might imagine that the Germans would have learned this during the interwar period from the MG13, which had substantially the same trigger. But they didn't, and kept the MG34 trigger group essentially unchanged throughout the war. They did have enough sense, however, to eliminate another complication, a clockwork rate reducer inside the grip, present in prewar models.
The real reason for the extra sear trip inside the cocking handle rail was to deal with feed stoppages in which bolt halted short of tripping the main release. Without the extra trip, yanking back the cocking handle to attempt to clear the jam would not release the sear, and the bolt could not be locked open. That's when you need three hands or a helper (which is about the same time that the bipod buckles and the muzzle drops into the dirt).
Been there, done that, for 50+ years.
M