FASA Games, Inc.

the spell says
- "each round the magician concentrates to make an effect test (WIL) against the target's willpower test."
what this means ?
the target does a WIL test, creating an effect number that the magician then has to beat with his WIL test ?

Yep.
The spell calls for an opposed test, basically.
Note that the target makes this test for free, and because it is a WIL Test to resist a spell effect, the target could use Willforce to resist (just as the caster could use Willforce on the Effect Test).

perfectly clear, thanks.
there arent any "special effects" based on the quality of the success for the caster ?
so the magician just need an average result

No special effects for Result Levels anywhere in there, they normally aren't necessary for spells where you have opposed rolls like this, because a high roll is good in itself.

Just for some history:
This spell has seen something akin to a major revision in every edition so far, and it's never been really satisfying for the majority, and was always one of the things most looked at. Some of the versions you'd have gotten an effect from a higher result level. All IIRC: For example, it initially also dealt a D6 damage vs. mystic armor, and an Excellent casting result would have made that armor defeating -- a D6 is pretty low even against mystic armor. Another version also had the damage, but you only got the paralysis if it actually caused damage (beat mystic armor), so again some use from armor defeating. Now you also got this resistance by the target, and no damage...
So this is a dangerous spell to mess around with. Just in case you decide you want to spice things up... ^^

thanks arma
well, first i want use it as it's intended.
there is a lot of room (read, time) to devise house rules but they can come only after a good use of the standard version