Aft Fuselage: Bucking Bar to reach under J-Stiffener angle?

On page 10-01, the Note at the bottom describes special tools while building the aft fuselage. It describes a "bucking bar able to reach under a J-stiffener angle". I am unclear on what this bucking bar needs to look like. What shape is ideal for this? Any insight on what others used to accomplish this is greatly appreciated.
Thanks!

I had to buy the small almost square tungsten bar with the very sharp angle on one face and this bar got into that space no problem. Big iron bars with the little angles stuck out foot work too. My long rectangular bar couldn't to it.

Thanks!

Excellent! Especially since I already have a footed bucking bar such as the Cleaveland BB30.
Thanks for the info....