It is not, strictly speaking a "channeling" skill. If you target a far monster and click you will spin to that monster--or spot were the monster was. That being said you can hold "stand still" and your attack mouse button and spin around by moving the mouse.
It does not matter whether the sword is main or off hand. The weapon specific skills only require that you have a weapon of that type equipped.
It only hits with the main hand. There are still some off hand properties you get; plus skills, oskills, -enemy resists, and a few others. If you are going to play a hybrid say deathgazer and windformer, then that is a good reason to duel wield.
Also note that currently windform still does not trigger procs of any kind. It will trigger on kill/death blow procs from poison damage kills IF you are not spinning at the time. This is not controllable enough to be reliable.