That is a parking lot safety brake lever. As l9ng as your brakes are bled it will activate the disc calipers. Pressing it rearwards activates brakes, releasing it disengages.
The stem bolt, sliding in the extended pin, should be almost full thread depth against the lock nut, with no threads exposed, forcing the pin to full slot depth as the coupler pulls backwards.
They will not engage on a hill against coupler pressure due to leverage. They retain position by going over and cam’ing against the frame.
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