From Ars Magica
The Chrono Anchor buff can be obtained by casting the Chrono Anchor Spell. When activated the buff saves the caster's location, inventory, health, mana, levels, and Magic Levels, resetting the caster to the saved state when the buff ends.
This buff will protect the caster from death, essentially ending the buff early so when the caster dies they reset to their saved location and keep everything they had when the buff was applied.
You can cast it while mining in order to prevent loss of items due to accidents like falling in lava or from a high place, or even if you fall into the void (if you use Blink or Seven League Stride you can pass through bedrock).
It's a good idea to use Chrono Anchor while you're fighting others players, because if the enemy player attacks you and finishes you off, you will not die and no items will be dropped or subsequently, stolen. The buff ID of Chrono Anchor is 46.