Every attack deals bonus random elemental damage at 100% modifier. Spirit procs less than the other three.
The elemental damage is considered an independent 'hit' on the target. It has an independent chance of being a critical hit or triggering an effect.
Damage of the elemental damage will match the damage of the skill being used. E.g.: If Mighty Blow deals 200% weapon damage, the bonus elemental damage will also be 200%. If Whirlwind deals 70% damage, the bonus damage will also be 70%.
Masterwork procs also benefit form this effect.
However, basic attacks deal half physical damage, but get the full elemental modifier (100% from Elemental, 50% Physical for a total of 150% weapon damage per swing).