Acceleration is Bravery's 2nd Epic Effect card added in Beta 11.0 during the UnderCards 1 year Anniversary. It was also the 2nd of the 2 leaked on Twitter. (The other being Last Dream.) Previous Versions can be found here.
When used, it will return all the cards in your hand back to your deck, followed by drawing 4 new cards. This can be helpful if you don't have any usable cards at the moment and want to try with a new hand.
This works surprisingly well if you end up having a full hand, few cards in your deck and cards on the field like Trader Temmie or Golden Flowers, as it will increase the cards in your deck and prevent you from losing cards due to a full hand.
At the cost of 2G, this is the 2nd Epic card to ever be lower than 3G (the other one being an older version of Napstablook). Inflation, Integrity's new spell, also turned out to be 2G.
At 1G, it is the first Epic card to ever be this cheap.