I'm not sure if I could find it again (it may in fact have been the Army Rigging Manual), but I did once find an explanation of force transfer through gin poles, sheer legs and tripods (It's pretty simple actually but the article was quite helpful).
TIMBEAL: The device you describe is a derrick crane (or near enough). Relatively simple to build and use (I believe) as long as you correctly manage the jib - pole connection.
http://www.oilyhands.co.uk/home_made_derrick_crane.htm
