ECU Power Source Circuit: Description
WARNING: This page is about a different car, the 2007 Lexus ES 350. However, it is still accessible from the selected car via links, so may be relevant.
When the engine switch is turned on (IG), voltage from the ECM's MREL terminal is applied to the EFI relay. This causes the contacts of the EFI relay to close, which supplies power to terminal +B of the TCM.