Fast Loot (for Raids) is a new option in v1.15. Here is the breakdown on the new functionality.

As you are running through, it will announce the Master Looted Items for the group to see.

Clicking on an Item name will start the normal roll process.

When the rolling is complete, select the +1 or +2 to the person you decided was the winner.

If you have Enable Auto Trade Popup enabled. The trade window opens up with the winner (IF IN RANGE) and drops the winning piece of loot into the window so you don’t have to dig through your bags.

When the trade completes, you will see an announcement that they completed the trade and the points are increased.
One the trade is confirmed, the item is removed from the Fast Loot Log. If there are multiple drops of the same item, it will be listed multiple times.