Now you need to rotate the knob downward 90º
Receiver is now unlocked, but keep holding the knob down with your right hand and grab the receiver with your left. Pull up and away. Tad bit heavy, but can be held with one hand. You can then see how it all fits together.
No receiver
Re-attachment is straight forward, just put it back into place. The red indicator will be showing.
Then with a mighty shove, push down and it rotates slightly into the locked position and the green indicator is showing. Lock it with the key.
This last part may take some initial attempts. I tried a couple of times rotating the knob to "help" it get into place, but that didn't seem to work.