The skill Bristleback is pretty straightforward. At any given time :
- If you are facing the source (understand the hero) of the damage : you don't get reduced damage
- If you're not facing the source of the damage : you get reduced damage.
It's not the projectile or the animation of the skill that matters but the position of the enemy hero and your position toward him. If Zeus casts Thundergod's Wrath at the other side of the map but you're not looking in his direction, you get reduced damage. As for damage over time, the same rule applies but separately for every ticks of damage. If you decide to man up and fight the Venomancer after he casted Poison Nova but realise you are undergoing way too much damage, you can litteraly turn your back on that fight in order to reduce the amount of damage taken from the DoT.