Asus AW171 - Manual (Page 29)

ASUS SK8N motherboard 2-9
2. Attach one end of the retention bracket to the retention module base.
3. Align the other end of the retention bracket (near the retention bracket
lock) to the retention module base. A clicking sound denotes that the
retention bracket is in place.
Make sure that the fan and heatsink assembly perfectly fits the
retention mechanism module base, otherwise you cannot snap the
retention bracket in place.