I read a bit about it once.
I know it deals with produce and events that can be triggered.
Some events I can think of:
Seeing Double (3 Shade at Domina. Easy to activate)
Pee Wee Birdie (Very hard to trigger. 3 Salamander and 3 Wisp at Domina. Pretty much requires the Sword of Mana artifact)
Wimpy Thugling (3 Dryad at Duma Desert)
Some enemies are strong against certain spirits while weak against others. Example: Rabites are strong against Wisp and Dryad, but weak against Shade and Aura. High Wisp and Dryad levels in lands with Rabites will make it stronger. It will become weaker if Shade and Aura are higher.
Some lands have demi-humans in Eggs, but to make them appear, you have to have a 3 of a certain Spirit to get it (example: Succubus appears in Fieg Snowfield if the Shade level is 3. Not related: Fieg Snowfield = awesome music)
Mana level also determines if you will find a wandering spirit in that area. According to FAQ, a Wisp won't appear in a land if there is no Wisp level.
I thought Tarosan was going to do Legend of Mana?