Like its predecessors, macOS Sierra supports spring-loaded folders. Such a folderpops open when you drag something onto it while holding down the mouse button. Spring-loaded folders work with all folder or disk icons in all views and in the Sidebar. Here’s how to use ‘em:
Select an icon (well, not a disk icon). Drag the selected icon onto any folder or disk icon while holding down the mouse button. The highlighted folder or disk will flash twice (very quickly), then springs open, right under the cursor.
(You can press the spacebar to make the folder open immediately.)
After the folder springs open, you can keep on “springing.” Subfolders continue to pop open until you release the mouse button.
Release the mouse button, and the icon you’ve been dragging is dropped into the active folder. That window remains open; however, all other windows you traversed close automatically.
To cancel a spring-loaded folder, drag the cursor away from the folder icon or outside the boundaries of the sprung window. The folder pops shut.
You can toggle spring-loaded folders on or off by going to System Preferences → Accessibility → Mouse & Trackpad. There’s also a setting for how long the Finder waits before it springs the folders open.