This assembly jig is the "bees knees" for putting frames together!
You put your end bars in the slots, put glue on, then drop the top bars in place. Lock the handle, and nail the top bars. Then flip it over and nail the bottom bars, loosen the handle, and you're done! It also provides space to nail the top bars from the side if desired.
Check out this video from our friend Bruce Jenne of the "Bruce's Bees" YouTube channel. He does a great job describing how this handy tool works!