As the book is not totally clear on this.
Fetishes are items.
“When learning a spell, a magician may choose to accept limitations on her ability to cast that spell. In exchange, the magician will have an easier time resisting the drain caused by the limited spell.” -SR4 P.172
This means that you can only cast this spell with the fetish and the fetish has to be touching your skin while casting the spell.
The fetish gives you a +2 dice pool to resist drain for a spell of your chosing, and one fetish per spell.
If you lose the fetish you can’t cast the spell till you find the old fetish, or buy a new one and pass an Magic + Intuition 5, 1 hour Extended Test.
One can of course buy this spell twice, one that uses the fetish, and one that doesn’t.