Whenever you play a Power, a random card in your hand costs 0 until it is played. The description states that it lasts until the end of the turn, but this is not in fact the case. If you play the card, then return it to your hand by whatever means, it no longer benefits from this cost reduction.
This relic gives rise to a unique archetype for the Defect, the so called powerbuild. Using cards like Heatsink, Storm and Creative A.I to make crazy combos. This strategy is explained in more detail on the Defect page under Power matters.
Weekly Patch 50: The Final Act
Fix for Mummified Hand relic targeting cards with costs lowered by Setup or Forethought.
Weekly Patch 23: Testing Continues...
Streamline now correctly reduces cost even when its cost has been lowered to 0 for the turn by something like Mummified Hand.
Weekly Patch 21: Channel Lightning
Seeds are now utilized for Matryoshka and Mummified Hand relics.