The default / no signal condition for the DC-DC converter is to charge the 12V battery ... converting HV to LV ... the car sends a signal ( comparatively high voltage on a single wire ) , to stop it from charging the 12V ... if you cut ( permanently or temporarily with a relay ) that control wire ( the wire signal goes to low ) you revert to the default condition of charging the 12V from the HV ... as long as the DC-DC convert is still connected to the HV.
The reason for the battery junction board is it is an easy way to keep the HV supplied to the DC-DC without significant car modification... If I recall correctly one of the control modules should also be unplugged... if there is no HV battery at all in the car.
Pictures and other descriptions can be seen on Mike's description of his use of the DC-DC enable for the FAS option for MIMA....
HERE.
- - - - - - - -
Another option ... if one wants to run permanently without the IMA battery ... the IMA motor always puts out AC power while it is spinning ... which it does whenever the ICE has any non-Zero RPMs ... If your only goal is to get alternator action for the 12V side ... you can do it with less weight and space than the module which includes the DC-DC converter and the IMA motor drive inverter.
This can be done a variety of ways ... from simple ( properly sized ) transformer and rectifier ... to more complicated switch mode AC-DC circuits ( which can be smaller , lighter, and more efficient than the simple transformer and rectifier , but is much more complicated ).
The Mechanical Load the IMA motor will pull down the drive line with ... will be a function of the electrical load pulled out ... always less than 100% energy efficiency conversion.