How can I run `git diff –staged` with Fugitive?

I’ve found a way to do this. Run :Git, you should get a window with contents like the following:

# Head: master
# Merge: origin/master
# Help: g?
#
# Staged (1)
# M example.txt
#

Scroll down to the staged file, example.txt, and press dd. This will open a diff view, comparing what’s in HEAD and what’s in the index. You’ll notice on the bar on the bottom that both the filenames are special Fugitive filenames.

Also while in :Git preview window, you can press g?, which will list all the mappings valid in the current context.