Let's you use a few special escape sequences in a line.
!$ expands to the last word in the previous line
!0 !1 ... !9 expand to the first ten words in the last line entered
!* expands to all but the first word in the last line entered
Use meta3-h to toggle function on and off.. ^K is bound to meta3 in this script, thus ^K-h will turn it on and off.