Q:

Using digraph mode to insert special key codes

I've always thought about digraph mode as a way to quickly insert unicode special or unusual chars into the buffer without copy-pasting things around or changing keyboard layout system-wide. So to insert an em-dash, for example, I just type <C-K>-M, to insert an ä, I type <C-K>a: and so on.
Today I stumbled upon another interesting thing you can do in digraph mode, namely inserting special key codes literally into the buffer. From the docs:
When {char1} is a special key, the code for that key is inserted in <> form.
For example, the string "<S-Space>" can be entered by typing <C-K><S-Space>.

I'm writing a post about Vim tips and tricks and this feature would help me a great deal to quickly enter literal commands. Unfortunately, it doesn't seem to work in my configuration. In insert mode, when I type <C-K><Tab> I would expect a <Tab> to show up on the screen, instead nothing happens.
Am I missing something here?
EDIT: I'm on OS X. My Vim build:
VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Jul 10 2015 19:36:53)
MacOS X (unix) version
Included patches: 1-712
Compiled by Homebrew
Huge version without GUI.  Features included (+) or not (-):
+acl             +farsi           +mouse_netterm   +syntax
+arabic          +file_in_path    +mouse_sgr       +tag_binary
+autocmd         +find_in_path    -mouse_sysmouse  +tag_old_static
-balloon_eval    +float           +mouse_urxvt     -tag_any_white
-browse          +folding         +mouse_xterm     -tcl
++builtin_terms  -footer          +multi_byte      +terminfo
+byte_offset     +fork()          +multi_lang      +termresponse
+cindent         -gettext         -mzscheme        +textobjects
-clientserver    -hangul_input    +netbeans_intg   +title
+clipboard       +iconv           +path_extra      -toolbar
+cmdline_compl   +insert_expand   +perl            +user_commands
+cmdline_hist    +jumplist        +persistent_undo +vertsplit
+cmdline_info    +keymap          +postscript      +virtualedit
+comments        +langmap         +printer         +visual
+conceal         +libcall         +profile         +visualextra
+cryptv          +linebreak       +python          +viminfo
+cscope          +lispindent      -python3         +vreplace
+cursorbind      +listcmds        +quickfix        +wildignore
+cursorshape     +localmap        +reltime         +wildmenu
+dialog_con      -lua             +rightleft       +windows
+diff            +menu            +ruby            +writebackup
+digraphs        +mksession       +scrollbind      -X11
-dnd             +modify_fname    +signs           -xfontset
-ebcdic          +mouse           +smartindent     -xim
+emacs_tags      -mouseshape      -sniff           -xsmp
+eval            +mouse_dec       +startuptime     -xterm_clipboard
+ex_extra        -mouse_gpm       +statusline      -xterm_save
+extra_search    -mouse_jsbterm   -sun_workshop    -xpm
   system vimrc file: "$VIM/vimrc"
     user vimrc file: "$HOME/.vimrc"
 2nd user vimrc file: "~/.vim/vimrc"
      user exrc file: "$HOME/.exrc"
  fall-back for $VIM: "/usr/local/share/vim"
Compilation: /usr/bin/clang -c -I. -Iproto -DHAVE_CONFIG_H   -DMACOS_X_UNIX  -Os -w -pipe -march=native -mmacosx-version-min=10.10 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1      
Linking: /usr/bin/clang   -L. -L/Users/haven/.sm/pkg/active/lib -fPIC -Bstatic -lz -fstack-protector -L/usr/local/lib -Wl,-headerpad_max_install_names -o vim        -lm  -lncurses -liconv -framework Cocoa   -fstack-protector  -L/System/Library/Perl/5.18/darwin-thread-multi-2level/CORE -lperl -framework Python   -lruby-static -framework CoreFoundation -lobjc -L/Users/zool/.rvm/rubies/ruby-2.2.1/lib   

A:

It will work only for (some of) the Vim 'special keys', literally - see :h ins-special-special. E.g. <C-K><Up>. Tab is not in that list. In addition, your terminal may not pass all key combos to Vim. For example, <S-Space> is there the same as <Space>, so it won't work (unlike in GVim). Long story short, your options seems limited, using this method.

