So, when should a shaman use his spirit binding skill to control a spirit?
I supposed that he could call it forth from its fetish to gain instantly its power (I thought that commanding a spirit was only needed for combat purpouses) . Should he make a succesful spirit binding to obtain its power?
Spirit Binding ties the fetch to the fetish. It is then used to connect the shaman to the fetch to he can draw on its powers. Thus, if I have a zebra spirit bound into my zebra mane fetish and I want to increase my movement (which is the spirit's power), I need to make a successful Spirit Binding roll to channel the power. Its not that the spirit it unwilling; but its akin to channelling any form of magical energy.
If a spirit that is bound to a fetish attempts to discorporate the soul of a living person (if that is what the Shaman commanded it to do) then, as Pete noted above, this can be done from the material or spirit world. But - discorporation and spirit combat can only occur on the spirit world (page 141).
A free spirit can only discorporate
from the Spirit Plane. A spirit tied to the mundane world - however that might be; fetish, magic or otherwise - can use its discorporate to drag an opponent's soul onto the spirit plane, as long as it has the Discorporate skill. Disease spirits, for example, are tied to the physical world through the physical representations of the their disease (the symptoms) and their worshippers, like Malia, in Glorantha.
So:
1) I don't understand Pete's comment that discorporation can take place from the material world, where a spirit is bound to a fetish (and so 'tied to the Material Plane') - can a bound spirit (that is tied to the material world) not actually use its discorporation power?
Yes it can: think of it as a version of enforced Spirit Walking. A high shaman can take other souls with him onto the Spirit Plane. Discorporation spirits can do precisely the same thing, but aren't shackled by rank.
A fetch with Discorporation can drag the souls of its shaman's enemies onto the Spirit Plane and batter the bejesus out of them; or hold them there until such time as the shaman commands the release.
2) This would make sickness or curse spirits effectively useless - if bound, and tied to the material plane, I don't see how they could discorporate another soul - spirit combat can only take place on the spirit plane?
Okay, imagine this scenario. Grunwald the Unwary is wandering through a broo tomb somewhere in Prax. He uncovers the rotten remains of Mistress Ghast, Malia High Priestess. Her horns are the fetish for her Creeping Chills disease spirit fetch. Grunwald, fancying some new horns for his helmet, seizes the skull...
The disease spirit immediately attacks, attempting a Discorporate. It matches its Discorporate 75% against Grunwald's Persistence 65% and wins. Grunwald's soul is now on the Spirit Plane, with the disease spirit, and about to go down with the nastiest disease of his life. As he has no Spirit Binding skill, he cannot resist combat and the within seconds he is completely infected with the Creeping Chills disease. On the material world his physical body begins to shake and shudder, exploding in horrid boils as the disease spirit takes effect. Within a few seconds, Grunwald is dead. On the spirit plane he had no magic to fend off the spirit, and no combat skill that he could use either. The disease is bound to the material world, but it can discorporate back to the spirit plane to wreck havoc.
This is what makes spirits so utterly, utterly nasty. Unless you have some anti-spirit magic, or a shaman, or a bound spirit of your own, you can be completely bleeding stuffed if a malevolent spirit successfully discorporates you. And bound spirits can do just that from the material world, returning to their binding afterwards to do it all over again.
3) Where in the rules is the ability to discorporate from the material or spirit world detailed
?
I do think this needs further clarification. Essentially its the same as Spirit Walking as cast by a high shaman: you resist the Discorporation with your Persistence in an Opposed test. Fail and you're dragged to the spirit world. Succeed and you're not. The spirit can make subsequent attempts to discorporate you just as with any form of attack.
4) Is a bound spirit prevented from moving back and forth between the spirit and material world?
A bound spirit resides primarily on the material world because its bound to a fetish or other binding object. It can discorporate to the spirit world to do its shaman's bidding but will return to its place of binding. It can return to the spirit world but simply can't do so at will.
Hope this clarified things a bit...