Bios < System boot code, low level hardware drivers, diagnosis software and encryption keys protection softaware. To crack the encryption protection software you connect wires from the mod into the bios. So while the PS2 copy the data from it to the RAM, the program code gets patched on the security code.
At the bios, while doing the diagnosis, it does test the CD/DVD controller chip (mecacon) so if it has bridged points the diagnosis will fail and you will get a black screen (blue if your TV has auto blue screen).
Taking the bios out might solve your problem, but it requires a bit more soldering skill than just soldering the wires. You might even end breaking the ROM chip (bios) like other guy here did before ...
SCPH-10000_GH-001 SCPH-15000_GH-003 SCPH-18000_GH-008 SCPH-30001_GH-005 SCPH-30000_GH-016(V4) SCPH-30001_GH-010(V4)
2xSCPH-10190, 2xSCPH-10350, 2xSCPH-10280
"**** j0 hackers!"
-Sjeep (As seen on TOXIC OS ELF...)