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!