Trample + Deathtouch
When you're assigning the damage that will be dealt by your Primeval Titan, you have to deal lethal damage to each blocking creature before you can deal any damage to the defending player. Normally, "lethal damage" means the defending creature's toughness minus whatever other damage it's already taken that turn. But if the creature dealing the damage (here, your Titan) has deathtouch, "lethal damage" is 1. Period. So your Titan has to deal 1 damage to your opponent's Titan. The rest (5) can then be assigned to the defending player. (It doesn't have to, though - you can still assign any amount of damage to your opponent's Titan if you want to.)