You can probably guess from the word hybrid!
They'll use the reliable old 2001 engines that they know all about to test new components in the engine. Perhaps a new gearbox, or something.
If they want reliability to test the new car, then they'll go for the old engine.
Sometimes they might run the old car with the new engine, etc, etc.
One thing to be certain about - they're not really going to be telling everyone about it, are they!