Asus NRL-LS - Manual (Page 33)

ASUS NRL-LS motherboard user guide 2-7
2. Position the fan with the retention mechanism on top of the heatsink.
Align and snap the four clasps on the retention mechanism over the
holes on each corner of the module base.
Keep the retention lock levers lifted upward while fitting the retention
mechanism to the module base.
Make sure that the fan and retention mechanism assembly perfectly
fits the heatsink and module base, otherwise you cannot snap the
hooks into the holes.
Retention Clasp
Snapped over
Retention Hole
Retention Lock Lever