HP zx6000 Processor Upgrade Instructions page 12

5. Use the special processor tool shipped with your replacement
processor assembly to lock the processor in place on the
system board. Insert the special processor tool into the hole
that runs down the side of the heatsink and rotate it clockwise
180 degrees.
Locking the Processor in Place
6. Slide the sequencing retainer plate toward the front of the
system.
Sliding Sequencer Retainer