Oh, just to stir things up a little...
I am using TWF and a Two-handed weapon as my main hand attack. My BAB is such that I get 3 iteratives plus 2 more off-hand attacks due to ITWF.
So, I make my 3 attacks with my main hand weapon.
Then as a free action, I remove my main hand from the weapon. Then as another free action, I place my main hand back on the weapon. I now make 2 attacks with my off-hand weapon which happens to be the exact same 2-hand weapon I have been using.
Definition of TWF: You can fight with a weapon wielded in each of your hands. You can make one extra attack each round with the secondary weapon.
For the pedantic would this be legal by RAW? If I was using a 1-handed weapon with 2 hands to start, and then switched, would that make a difference? What if my off-hand attacks only used it 1-handed?