Jump starting Yaris Hybrid
First of all, Yaris Hybrid can't be jump started like the conventional cars. But what you most likely need to do is to supply little energy to 12V system so the HSD get it's computers up and the high voltage circuitry energised. Normally the problem is your 12V battery has gone flat so there is no energy to switch the high voltage side contactors/relays on. If the high voltage battery is flat you probably need to contact Toyota service to come over with some special 144V charger or boost pack.
I have had to feed the 12V system for several times to get going. Usually I do it from the fuse box on the engine bay, but the normal jump start cables won't fit there so you need to have small alligator clips instead. Second easiest way to do the jump start is simply to remove the back seat, under which both of the batteries are located. It's not as complicated it sounds, you just need to find the two fix points in front edge of the seat, and pull hard enough. You could also access the battery by removing the plastic cover on the right side legroom, but I found it harder to put back in place than the seat.
The system doesn't need lot of energy to get computers started, I use a small 2Ah sealed lead battery.
By the way, Toyota has developed a real battery killer in the system. You may have noticed the brake booster will start immediately when you open the driver side door. It is normally fine, but if your battery is nearly empty, the booster will make sure it dead before you get a chance to turn the system on. And the best of all it will keep trying to pump up the brake pressure no matter the battery has gone flat long ago. You will hear a whining from the engine bay, and there is no way that I know to stop it. The good news is that if you open the passenger side door, the pump won't be activated. So if you know or fear your 12V battery is nearly empty, crawl in from the passenger's door and try your luck to get the car turned on.
In my Yaris Hybrid version there is a Start button, so the procedure to get the system up as quickly as possible is to push the brake pedal and immediately after the Start button. If you don't push the brake pedal, the system does not engage the high voltage contactors, and will continue to suck juice from the 12V battery.
The 12V battery is normally charged from the high voltage side.
Picture: 12V 35Ah battery in front. The two white plastic slots keep the back seat in place. 144V 6.5Ah high voltage battery in the middle.