I run the TIU in active mode with power going through it. That gives the Remote emergency power off capability, and also I get a slightly better DCS signal level.
Passive mode is connecting the transformers directly to the track and connecting the TIU outputs directly to the track, no input power in the TIU inputs. However, unless you're SURE that FIXED #1 will ALWAYS have power, you need AUX power into the AUX Power jack.