ctrl-r <register> pastes that register into your command.

So I often need to find "where is this method defined" across a large project.

    yw (yank word to get method into the " register)
    :Ack <ctrl-r>"  (ack plugin, paste that default " register)
    <enter> yay! I found it.

Alternately:

    /foo  (attempt to find foo in this file. Whoops, not there, where is it!).
    :Ack <ctrl-r>/  (last search is in the / register).
    There it is!