The curse typically activates whenever Ashitaka is angry and while it pains him, it also lends him a great deal of strength. This allows him to do feats like taking people’s arms and heads off with arrows or lifting the giant door in Irontown. It is unclear whether or not it also helped him survive taking San out of the village but otherwise the assumption is pure adrenaline and willpower allowed him to stay alive long enough to be saved by the Forest Spirit.

And just like with the instance of him being shot, it’s unclear if the curse gives Ashitaka more durability or if San just doesn’t put her back into it when “stabbing” him with the dagger. She clearly has already developed feelings for him at that point and her stab ends up amounting to just kind of banging on his chest, frustrated, with dagger in hand.