palpitation explanation?

I got what i think called palpitation a lot. (heart beating hard even when it's not fast) and i noticed many people experience it too and be told to just deal with it. But i was wondering what actually happens when the heart beating really hard like it's punching out of your chest/about to explode. especially when it's really bad. it's clearly, obviously and totally different than when your heart is beating calmly/normally.
Till now i haven't found out how it's doing when it happens/what makes the differences.
Is it detectable with holter monitor? Or have to be with Echo? And is it from the heart, nervous system? (isn't the electrical problems only involve HR but not the hard beating even when the HR is considered in normal range)?
Does anyone know an explanation about it? Tweet