There are no forcefields in Blades of Death (BoD). If you're referring to the fan, you simply drop the the lower level that the fan is on, go through either door on that level, follow the route to the health/shields and you will see a button. Activate that button and it will turn off the fan. You may then drop in and get the Rail Detonator or the Concussion Rifle, but beware...the fan comes back on in around 15 seconds or so.
If you're referring to Nar Shaddaa Loading Terminal (NSLT), which does have a forcefield, you simply go to the upper floor of the level, by going to the ramp beyond the conveyor belt, up the elevator, down the corridor with the gas tanks, through the 2 doors, and to the switch that will be straight across from you there. That switch will allow you to open and close the forcefield. There is also a switch on the catwalk, outside the forcefield, that enables you to turn the forcefield on and off as well.
I hope that helps. Have fun.