Subject to change, but currently:
Item - Priest T20 Discipline 2P Bonus (New) The first bolt of light from your Penance deals an additional 200% damage.
Item - Priest T20 Discipline 4P Bonus (New) Power Word: Shield has a 25% chance to cause your next Penance to be free and have 50% reduced cooldown.
Since we're already casting Penance as near as possible to CD, the 2-piece is just a boost to Penance's power and has no gameplay effect. The 4-piece mostly likewise - we're already casting PWS near CD and the additional Penances from the cooldown reduction will mostly replace Smites/empty GCDs.
There's one gameplay change here though in raids, for players who currently don't cast Penance during Rapture - casting Penance during Rapture is greatly boosted in power, so it will now be "required".
This free mana spell will both help the spec become more active (as free mana Penance is used sometimes where an empty 1.33 GCDs used to be) and will reduce the mana required per minute to play Disc (as free mana Penance sometimes replaces Smite).