Well, yeah, that's the car's KERS system (electric boost, you might already know that). The F1 cars have it as well. The other bar is probably DRS, which I believe all three hybrid supercars have in real life, but in-game still don't let us use it.
Also, little helpful English lesson, it is much more correct to say "I have to hold the R2 button all the time". What you said still makes sense, but is sort of like Yoda-speak, it is a bit backwards. I only offer to help because I'd hope Deutsch speakers will do the same, since I casually try to study it.
Gute nacht or guten morgen depending on when du read this!