Activating a Magic Item:
Activating the magical powers of most magic items takes an instant action unless that item is a passive item. Unless otherwise stated each item is one use per combat. This applies to the item not the person so an exchange during combat would still not allow the item to function even if all the other criteria were met. Natural abilities of an item such as its use as a weapon for example is obviously not affected by this rule.
Sacrificing - Activating an item for the first time sometimes takes a sacrifice of a totem or other item of value.
Resistance: you gain a % chance of getting another roll.