From LoTROInterface Wiki
Function to handle moving or swapping an item in our inventory. This DOES NOT handle dragging items from any of the character banks or housing hooks. For that functionality please use PerformShortcutDrop.
|function Backpack:PerformItemDrop(item, destinationIndex, split);|
The LuaItem representing the item you are trying to move
The index into the backpack where you are trying to place the item
Whether or not we want to split the item's stack while dragging it