If I'm not mistaken, the 12v battery, which is quite small in capacity, does not have available to it the power in the IMA battery to maintain it's charge level unless the key is in the "run" position. A long coast with the lights and accessories on, but the engine off, can quickly run down th 12v battery to a point where it no longer can support the onboard electronic brain, and strange things begin to happen. One of these things will be the IMA battery suddenly displaying NO charge. (There still is plenty of juice in the IMA battery, but the electronic brain can no longer ascertain that fact.)

When the 12v battery dies, jump starting the HCH simply entails repowering the electroinic brain so that when the key is turned to the run (or start) position, the IMA power again becomes available. It is the power in the IMA battery which powers starts and restarts of the engine, not the 12v one. Also, the depleted 12v battery will be recharged automatically, and quite quickly, courtesy of the IMA battery, as you drive. It's quite an elegant system!