Apple PC Compatibility Card - Manual (Page 6)

6 Chapter 1
5 Touch the metal part of the power supply case inside the computer.
Always do this before you touch any parts, or install any components, inside
the computer. Touching the power supply case helps release static electricity
from your body and prevent possible damage to internal components.
IMPORTANT You can collect static electricity just by walking away from and
then back to your computer. If you need to move away from your computer
during installation, remember to touch the metal part of the power supply
case again before you continue installing.
Power supply case