Bond With Nature restores your Health whenever one of your Animal Companions is killed or unsummoned. The Feral Guardian is the only companion that can actually be killed, but this will still work with the other skills - Dive and Scorch will both heal you when they hit, and Swarm, Betty Netch, and Falcon's Swiftness will heal you when they expire. All of these are delayed effects, so it's not extremely effective as a means of self-healing compared to dedicated healing spells such as those in the Green Balance line or Frost Cloak, but being able to attack and heal at the same time is very helpful.