Well it's amazing that the harrier can even be hovered. It basically stands on four legs. It's a balancing act it has 4 exhaust ports. 2 of which are more under the CG that the more rearward ones. These controll the main forward flight to hover, and vice versa. The other two help to keep it level, and pivot with rudder inputs I think to rotate the aircraft in a hover. Which is all done manually through controlls in the cockpit by the pilot. Basically if you look at a harrier it has two large extakes just forward of the CG, and two smaller ones further back. The mains provide the hovering force yet the airframe is slightly tail heavy in hover so thrust from the smaller rear extakes provide just enough thrust to keep it level. There's 2 on each side which keeps the lateral balance.
It's extremely inneficient, and not all too manuverable in a hover. The biggest problem is the huge amount of thrust you lose with the ducting. I belive you lose about 40% of the thrust.