As seen in the above picture, I have installed two LEDs in the dash that indicate when the ECU switches the TVIS (Toyota Variable Induction System) and VSV (Vacuum Switching Valve). The red LED turns on when the ECU provides ground to the TVIS pin on the ECU. This indicates that the ECU has opened the TVIS ports. This is a useful feature as the ECU will open the TVIS ports under any throttle position other than idle (closed throttle) if it detects bad gas or excessive engine knock. During normal operation, the TVIS ports *seem* to be opened depending on RPM and the signal from the air flow meter (AFM). I see the ECU opening the TVIS ports as early as 2500RPM if the throttle is opened far enough. It will keep the TVIS ports opens under any throttle position or AFM signal if the RPMs are above ~4500RPM. I've found that I can up the boost/play with water injection/different injector sizes etc. and keep an eye on how the TVIS LED responds and hence find if the ECU goes into bad gas mode (ie too much knock).
The VSV LED is connected to the VSV pin on the ECU. Connected in this manner (probably not the best way), the VSV LED will stay on until positive boost is reached, and if the ECU deems it is OK to run full boost. If the intake air temperature drops below about 40 degrees, the ECU will not allow full boost, and hence the LED will stay on all the time, regardless of boost pressure. The ECU will also not allow full boost (LED on all the time) if it goes into bad gas mode. So under normal conditions the VSV light should be out under wide open throttle (WOT).
When connecting the TVIS LED, it is best to connect one end of the diode to +12V, and the other to the TVIS pin on the ECU. Connected in this manner, the LED will light when the TVIS ports are closed. It will turn off when they are opened.
I've found that there seem to be degrees of knock. In some cases while upping the boost pressure, I've seen the ECU go into bad gas mode (TVIS LED off under any throttle position other than closed), and then after about 10 minutes of driving go back to normal mode. In other cases, I've seen the ECU go into bad gas mode and stay in the mode until the engine is restarted, and driven. The ECU will also enter this mode after an ECU reset. If the ECU is reset (remove the EFI fuse for ~20 seconds), the car must be driven for about 10 minutes before the ECU switches to normal operating mode. For more information check out the TOYOTA DOCUMENTS page.