This means that git reset -p is the opposite of git add -p (see git-add(1)).

git reset --option target

working index HEAD target         working index HEAD
----------------------------------------------------


 A       B     C    D     --soft   A       B     D


                          --mixed  A       D     D


                          --hard   D       D     D


                          --merge (disallowed)


                          --keep  (disallowed)

working index HEAD target         working index HEAD
----------------------------------------------------


 A       B     C    C     --soft   A       B     C


                          --mixed  A       C     C


                          --hard   C       C     C


                          --merge (disallowed)


                          --keep   A       C     C

working index HEAD target         working index HEAD
----------------------------------------------------


 B       B     C    D     --soft   B       B     D


                          --mixed  B       D     D


                          --hard   D       D     D


                          --merge  D       D     D


                          --keep  (disallowed)

working index HEAD target         working index HEAD
----------------------------------------------------


 B       B     C    C     --soft   B       B     C


                          --mixed  B       C     C


                          --hard   C       C     C


                          --merge  C       C     C


                          --keep   B       C     C

working index HEAD target         working index HEAD
----------------------------------------------------


 B       C     C    D     --soft   B       C     D


                          --mixed  B       D     D


                          --hard   D       D     D


                          --merge (disallowed)


                          --keep  (disallowed)

working index HEAD target         working index HEAD
----------------------------------------------------


 B       C     C    C     --soft   B       C     C


                          --mixed  B       C     C


                          --hard   C       C     C


                          --merge  B       C     C


                          --keep   B       C     C

working index HEAD target         working index HEAD
----------------------------------------------------


 X       U     A    B     --soft  (disallowed)


                          --mixed  X       B     B


                          --hard   B       B     B


                          --merge  B       B     B


                          --keep  (disallowed)

working index HEAD target         working index HEAD
----------------------------------------------------


 X       U     A    A     --soft  (disallowed)


                          --mixed  X       A     A


                          --hard   A       A     A


                          --merge  A       A     A


                          --keep  (disallowed)

$ git commit ...
$ git reset --soft HEAD^      (1)
$ edit                        (2)
$ git commit -a -c ORIG_HEAD  (3)

1. This is most often done when you remembered what you just committed is incomplete, or you misspelled your commit message, or both. Leaves working tree as it was before "reset".
2. Make corrections to working tree files.
3. "reset" copies the old head to .git/ORIG_HEAD; redo the commit by starting with its log message. If you do not need to edit the message further, you can give -C option instead.

See also the --amend option to git-commit(1).

$ git commit ...
$ git reset --hard HEAD~3   (1)

1. The last three commits (HEAD, HEAD^, and HEAD~2) were bad and you do not want to ever see them again. Do not do this if you have already given these commits to somebody else. (See the "RECOVERING FROM UPSTREAM REBASE" section in git-rebase(1) for the implications of doing so.)

$ git branch topic/wip     (1)
$ git reset --hard HEAD~3  (2)
$ git checkout topic/wip   (3)

1. You have made some commits, but realize they were premature to be in the "master" branch. You want to continue polishing them in a topic branch, so create "topic/wip" branch off of the current HEAD.
2. Rewind the master branch to get rid of those three commits.
3. Switch to "topic/wip" branch and keep working.

$ edit                                     (1)
$ git add frotz.c filfre.c
$ mailx                                    (2)
$ git reset                                (3)
$ git pull git://info.example.com/ nitfol  (4)

1. You are happily working on something, and find the changes in these files are in good order. You do not want to see them when you run "git diff", because you plan to work on other files and changes with these files are distracting.
2. Somebody asks you to pull, and the changes sounds worthy of merging.
3. However, you already dirtied the index (i.e. your index does not match the HEAD commit). But you know the pull you are going to make does not affect frotz.c nor filfre.c, so you revert the index changes for these two files. Your changes in working tree remain there.
4. Then you can pull and merge, leaving frotz.c and filfre.c changes still in the working tree.

$ git pull                         (1)
Auto-merging nitfol
CONFLICT (content): Merge conflict in nitfol
Automatic merge failed; fix conflicts and then commit the result.
$ git reset --hard                 (2)
$ git pull . topic/branch          (3)
Updating from 41223... to 13134...
Fast-forward
$ git reset --hard ORIG_HEAD       (4)

1. Try to update from the upstream resulted in a lot of conflicts; you were not ready to spend a lot of time merging right now, so you decide to do that later.
2. "pull" has not made merge commit, so "git reset --hard" which is a synonym for "git reset --hard HEAD" clears the mess from the index file and the working tree.
3. Merge a topic branch into the current branch, which resulted in a fast-forward.
4. But you decided that the topic branch is not ready for public consumption yet. "pull" or "merge" always leaves the original tip of the current branch in ORIG_HEAD, so resetting hard to it brings your index file and the working tree back to that state, and resets the tip of the branch to that commit.

$ git pull                         (1)
Auto-merging nitfol
Merge made by recursive.


 nitfol                |   20 +++++----


 ...
$ git reset --merge ORIG_HEAD      (2)

1. Even if you may have local modifications in your working tree, you can safely say "git pull" when you know that the change in the other branch does not overlap with them.
2. After inspecting the result of the merge, you may find that the change in the other branch is unsatisfactory. Running "git reset --hard ORIG_HEAD" will let you go back to where you were, but it will discard your local changes, which you do not want. "git reset --merge" keeps your local changes.

$ git checkout feature ;# you were working in "feature" branch and
$ work work work       ;# got interrupted
$ git commit -a -m "snapshot WIP"                 (1)
$ git checkout master
$ fix fix fix
$ git commit ;# commit with real log
$ git checkout feature
$ git reset --soft HEAD^ ;# go back to WIP state  (2)
$ git reset                                       (3)

1. This commit will get blown away so a throw-away log message is OK.
2. This removes the WIP commit from the commit history, and sets your working tree to the state just before you made that snapshot.
3. At this point the index file still has all the WIP changes you committed as snapshot WIP. This updates the index to show your WIP files as uncommitted.

See also git-stash(1).

$ git reset -- frotz.c                      (1)
$ git commit -m "Commit files in index"     (2)
$ git add frotz.c                           (3)

1. This removes the file from the index while keeping it in the working directory.
2. This commits all other changes in the index.
3. Adds the file to the index again.

$ git tag start
$ git checkout -b branch1
$ edit
$ git commit ...                            (1)
$ edit
$ git checkout -b branch2                   (2)
$ git reset --keep start                    (3)

1. This commits your first edits in branch1.
2. In the ideal world, you could have realized that the earlier commit did not belong to the new topic when you created and switched to branch2 (i.e. "git checkout -b branch2 start"), but nobody is perfect.
3. But you can use "reset --keep" to remove the unwanted commit after you switched to "branch2".