Asus M3N78-EH - Manual (Page 24)

1-12 Chapter 1: Product introduction
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.