Describe the purpose of your piece of technology, from the perspective of your non-advanced culture by explaining what you want it to be able to do. Use basic terms if your are unsure, such as saying that it is a weapon, a tool or a mode of transportation. For instance, if you decide that your non-advanced culture would see the item as a weapon, then you may decide that it will be a melee weapon that your character must hold in his hands to use.
Assign an advanced purpose to your new weapon, from the perspective of your advanced culture and explain specifically what they would use the item for. Relate the use of the item to something familiar, something unfamiliar or both. As an example, you may decide that your melee weapon is actually an artistic tool, designed to reshape metal through imperceptible vibrations. Use familiar connections to show a difference between the cultures, such as the advanced artist tool and the non-advanced tool of combat. Select unfamiliar connections to establish your advanced culture as more exotic, utilitarian or advanced, such as the reshaping of metal through imperceptible vibrations would be very advanced.
Ask yourself how the item may have been changed in its path to the past by defining specific ways that it was changed and the result of those changes. For instance, you may decide that your artist's tool was cracked during its trip to the past and now has a long sharp edge along one side, changing it from its formal purpose and into an acceptable non-advanced weapon.
Define the specific advantages that your technology has for your non-advanced culture by explaining the "magical" elements of your piece. Assign additional properties due to the item's original purpose but make sure to keep these assignments discrete and unrevealing. As an example, you decide that your non-advanced sword still has its imperceptible vibrations, moving armor out of its way as it strikes its enemies and even cutting through enemies with ease as a further result of the vibrations. Additionally, your sword may leave beautifully ornate cutting patterns in armor, doors or enemies as it cuts.