Asus M4A79XTD EVO/USB3 - Manual (Page 28)

2-8 Chapter 2: Hardware information
Chapter 2
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.
Ensure that the fan and heatsink
assembly perfectly fits the retention
mechanism module base, otherwise
you cannot snap the retention bracket
in place.