Magnetic fields are like disembodied magnetic forces. These are typically emanated from emitters bolted onto the walls, ceiling or floor, and look like a force flowing out from these emitters and away from them in a straight line.
Objects charged with the same colour as the beam itself will have force added to them in the direction that the field seems to be flowing. Objects with the opposite colour will flow towards the stream.
The force of the stream is uniform in the entire area it covers, but objects only partially inside the stream only have force added to the part of them that is inside the beam itself. If the player for example is only halfway immersed in the stream, he will only get half as much force as he would if he was completely immersed. For objects that can rotate this causes the part of them that is within the beam to be moved, while the part outside the beam is dragged along, causing rotation.