pete914, is the Z620 jumper you are asking about located between the white PCI-32 connector and the black PCIe connector above it? Is it the jumper highlighted in yellow in the picture below?
If so, this is the E15 Boot Block recovery header. There is a jumper on pins 2 and 3 (right hand side pins in this picture).
Moving the jumper from pins 2-3 (default) to 1-2 manufally forces the system into boot block recovery mode. Starting the system with the jumper on pins 1-2 will cause the system to try to reflash the system BIOS from an attached USB drive. It will appear like the system is not booting, and there will be some keyboard flashes and maybe some screen prompts (It's been a very long time since I tried this).
If the system BIOS ROM is corrupted, the system will automatically go into boot block recovery mode. It can be forced into this mode by moving the jumper and then rebooting the system. If you system boots normally, do not move this jumper. Leave it alone.
Does this answer your question?