When using lazy loading of Ex command(s), the Ex commands should be defined by volt bootstrap script (~/.vim/pack/volt/start/system/plugin/bundled_plugconf.vim).

Here is a example of lazy loading of Ex command(s):

" In $VOLTPATH/plugconf/github.com/tyru/open-browser-github.vim.vim
function! s:loaded_on()
  return 'excmd=OpenGithubFile,OpenGithubIssue,OpenGithubPullReq'
endfunction

Defined Ex command should accept:

• arguments
• range
• bang
• completion (<Tab> loads the plugin immediately)
• of course, executing the Ex command loads the plugin and execute loaded command

Ref

on_cmd feature of Shougo/dein.vim