Asus M4A89GTD PRO/USB3 - Manual (Page 26)

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 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.