Asus M2N4-SLI - Manual (Page 28)

2-8 Chapter 2: Hardware information
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.
4. Push down the retention bracket lock on the retention mechanism to
secure the heatsink and fan to the module base.
Make sure that the fan and
heatsink assembly perfectly
fits the retention mechanism
module base, otherwise you
cannot snap the retention
bracket in place.