The Multi-Spatial force field was a type of force field used by One to attempt to survive the implosion and destruction of a Borg Sphere. He had directed its flight path into a nearby nebula, thus saving the crew of the USS Voyager from assimilation. While the force field was able to protect him enough to be beamed to Voyager, he still died after injuries he sustained on the Borg Sphere. (VOY: "Drone (episode)")
Media:Drone_anim.gif - Click to watch
Speculation
It is speculated that Multi-Spatial force fields are generated by millions of specialized nanoprobes, within One's body after he was created from 29th century mobile emitter technology.
Each nanoprobe would emit a portion of the shield grid, which by the sum of all of the specialized nanoprobes, envelopes One's entire body.
By anyalizing a threat through his holographic imaging eyepiece on his ocular implant, One would be able determine which type of shielding would be required to defend himself.
Since these nanomachines were created out of mobile emitter technology, One could also create holographic projections (i.e. sensor ghosts, other beings, stealth camouflage, etc..) in order to decieve and attack enemy threats (though not shown in the episode).