Just noticed this today.
If you switch between your different presences when you are low on health after fighting mobs you can get your health back quickly. No need for food or bandages.
Switch from frost to blood or unholy and back to frost.
When you switch from frost to one of the others you will go from a lower percentage of health to a higher percentage. Since your frost presence gives you more HP
Eg In Frost 1200/9100 switch to blood you are now 1200/8000 = higher percent switch back to frost and you are now healed at the same percent as you were in blood.
Then when you switch back to frost you will stay at that higher percentage. Switch back again and so on until you heal fully.