Each PC gets one Hero Point per character level that they can use per gaming session. A HP can be used to increase a PC's roll by 1 point per HP spent, or can increase the damage done by an already successful attack by 1 point per HP spent. Alternatively, a PC can spend HPs to reduce the attack roll and/or damage of an opponent, as long as the attempted attack or action directly affects them.
HPs are generally the domain of Player Characters, but the GM may choose to occasionally give an NPC some HPs as well, to perhaps stretch out any fights that might occur, and to give the NPC in question a chance to avoid getting dropped by one lucky shot. That said, encountering an NPC with HP should be a fairly rare occurrence; a 'standard' dungeon setting should probably only have one NPC with HP (and perhaps a favored henchman who has HP equal to half their level). In a 'megadungeon', one NPC with HP might be a good rule of thumb.
The use of Hero Points isn't terribly 'old school' in feel, and not allow PCs to have HP would not be too out of place. That said, as the combat system for Trollslayers is pretty damned harsh and unforgiving, and the use of any given HP has only a minor effect, allowing the PCs a chance to affect their chances in small ways shouldn't alter the style of play overly much.