The big sword is a Ki manifestation of the second sword on his back. I imagine that sword and the use of it are supposed to be linked in to how his abilities work in general. When using his Q he actually draws the second sword and uses it to attack, where as he usually only attacks with the smaller one. In my head I'm imagining this as the second sword being a means for Shen to focus his Ki outwardly in to a physical form to then use as a means of aiding himself in combat. We know sod all about Ki in League so those are just my thoughts.
Having played him on the pbe I can say that I actually kinda enjoy it; playing around the sword and what it provides you is key to being good at Shen, and it gives you a good means of doing things; you Shadow Dash in to line up your Q, use your Twilight Assault to summon your sword, cutting through the enemies and then do extra damage because you did so in the correct way. You use Spirit Refuge in all of this to keep yourself safe.