Asus P4S8X User Manual page 27

2. Position the fan with the retention mechanism on top of the heatsink.
Align and snap the four hooks of the retention mechanism to the holes
on each corner of 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 Lock
Keep the retention locks lifted upward while fitting the retention
mechanism to the module base.
ASUS P4S8X motherboard user guide
Retention Hole
Retention Hook Snapped
to the Retention Hole
15