# This information was provided by doener from irc.freenode.net#git
# Many thanks go to him!
 
A---B---C---D (rightbranch)
 \
  E---F---G---H---I---J (wrongbranch)
 
 
You did:
git checkout wrongbranch
git rebase --onto rightbranch HEAD~6
 
Which is equal to:
git rebase --onto rightbranch HEAD~6 wrongbranch
 
and results in:
              E'--F'--G'--H'--I'--J' (wrongbranch)
             /
A---B---C---D (rightbranch)
 \
  E---F---G---H---I---J
 
 
 
You wanted:
git checkout rightbranch
git reset --hard wrongbranch
git rebase --onto rightbranch@{1} HEAD~6
git checkout wrongbranch
git reset 
 
The reset --hard wrongbranch gives us:
 
A---B---C---D
 \
  E---F---G---H---I---J (wrongbranch) (rightbranch)
 
And the reflog entry rightbranch@{1} now references D (previous state of rightbranch, before the reset)
 
Thus:
git rebase --onto rightbranch@{1} HEAD~6
Equals:
git rebase --onto D A rightbranch
 
So that gives:
 
A---B---C---D---E'--F'--G'--H'--I'--J' (rightbranch)
 \ 
  E---F---G---H---I---J (wrongbranch)
 
So what's left is to fixup wrong branch, and that's what the remaining checkout + reset commands do.