I also saw the note about "Major update to damage scaling"
So If I understand, each HD allows to do an action, and an action can be an attack. Each additional action can be an attack or something else. Is it correct?
That is correct!
So for example, if you have a 4 HD character and he does an attack and focuses all his HD on the one attack, he will use 1 HD to do the attack and if it succeeds, will deal damage + 3 (which would be the extra actions utilized in that attack).
He can also do attacks to two opponents (using 2 HD) and have 2 HD left over to distribute as additional damage. (+2 to one, +1 to each, etc) Hope that makes sense!