This seems challenging for a non-native like me to understand. I sort of anticipated this would have been the idea, but it's still hard to see if that's what it actually means - it's hard to see if 'drop' is a noun or a verb. It would be easier if it had a context word like you give here, i.e. "idea, project or reference".

Anyway, it's an interesting UI idea, particularly the mouse-oriented sorting of items. I wish a middle ground between this and something like checkvist or dynalist could be found.