That is the "HK Slap".
The term can be found in the HK MP5 Operator's manual.
The correct term for that is "cocking lever".
There are two ways to load a HK MP5.
Adminstrative loading (chamber clear, cocking lever locked to the
rear, safety on)
Insert magazine into magazine well.
Tug on magazine to ensure it's fully seated.
With the non-firing hand. slap the lever out of the notch in ONE
motion.
DO NOT RIDE THE LEVER FORWARD OR THE ROUND MAY NOT FULLY
CHAMBER
Tactical reloading:
Note 1: a 30 round magazine overfilled with 31 rounds cannot be
seated in the well with the bolt forward.
Note 2: If the action is not carried out with aggression, a
30 round magazine may prove difficult to seat in
the well with the bolt forward.
Transition to back up weapon if immediate threat is present.
Stay alert.
Signal partner that you need to reload.
Seek cover.
Tilt, rack, and lock.
Secure loaded mag (BEFORE removing)
Remove empty/half empty magazine.
LOAD.
slap lever forward.
Move on.