Spirit shield is an ego only found on hats, scarfs, the hat of the Bear Spirit, and some randarts. This ego duplicates the effect of an amulet of guardian spirit, linking your HP and MP pools so that all damage you take will be split proportionally between them. This even affects normally hard-to-resist damage sources such as torment and smiting. Should your MP reach zero, damage will be taken entirely from your HP until at least one MP is restored.
Immediately upon putting on a spirit shield item, your current MP will be reduced to zero, but in most cases it will regenerate as normal. Deep dwarves behave differently; their MP stops regenerating, but MP channeling and other methods of restoring MP still function normally.
Damage Reduction Example
If you have 200 HP and 20 MP, an 11 damage attack will reduce your HP by 10 and your MP by 1. Characters with more balanced HP:MP ratios will see much more dramatic reductions to damage.