Asus AP120-E1 - Manual (Page 34)

2-8 Chapter 2: Hardware information
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.
Keep the retention locks 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 Hole
Retention Hook Snapped
to the Retention Hole
Retention Lock