Eidos has held back on explaining a couple of features of Lara Croft and the Guardian of Light until yesterday at a showing in Santa Monica. We saw that the item-based power up system uses both Artifacts and Relics, found items that can be applied to your characters to enhance their abilities and stats.
All of the Artifacts are assigned to different statistics, and some can combine more than one. You can equip them and use them to boost your character, giving them different powers than their co-op partner. For example, one artifact could add +2 to your speed, making your character move around faster. Another could be both additive and subtractive, adding to speed, but taking away from something like defense. You can equip two at a time, affecting up to four stats at once.
Relics are like Artifacts, but relate to abilities rather than stats. Relics can give you more powerful shots, larger bombs, ammo regeneration, health regeneration and more. Unlike Artifacts, you'll only have access to your Relic powers by having your power meter charged. Something to note is that you'll only have access to your Relic powers and your power bar as long as you do not take damage. In other words, you'll really have to watch your ass if you want to do some real damage.
Check out Hamza's recent preview for more on Lara Croft and the Guardian of Light.