It was the way HK used to teach MP5 reloads.
Grab NEW mag; bring action (working parts) BACK (to the rear); remove old mag = OFF; magazine in, seat and tug = ON; working parts FORWARD
With the clamp you already have the new mag so it omits the NEW.
I thought it was lock the bolt to the rear, dump the old mag, grab the new mag and lock it, then let the bolt forward. I'll practice the "right" way.