No pun intended, I promise.
Rocket League's new free update provides mainly the new Rumble mode, described as a battle royale mode with players being randomly assigned one of the eleven new powerups.
It comes with a bit more, however. Primarily, the SteamOS and Mac versions of the game are available as of this update. There are a few known issues, but Psyonix is working to fix them. Party chat has also been enabled during the game, and players will now have 10 customization slots for their vehicles.
Finally, a random loot system, seemingly not unlike CS:GO's, is added, in which players can buy keys to unlock random rewards.
For the full blog post from Psyonix, go here.