That is how to move a trailer (2,3)
I believe the answer is:
to wit
'that is' is the definition.
('to wit' can mean 'namely' or 'that is')
'to move a trailer' is the wordplay.
'move a trailer' becomes 'wit' (I can't explain this - if you can you should believe this answer much more).
'to'+'wit'='TO WIT'
'how' acts as a link.
Can you help me to learn more?
(Other definitions for to wit that I've seen before include "specifically" , "what to do with a broken-down car!" , "i.e." , "In other words" , "Namely" .)