When you look at that picture from 1952 (courtesy Draeger) you see that bouyancy- control is no black magic to rebreather divers, it is just knowhow and training.
On an CMF-SCR the Nozzle fills the counterlung(s), all the diver has to do from time to time is dumping the amount of gas which is too much. It depends on the model if the dumping is done by exhaling through the nose, stooping or doing something else. When were is not enough buoyancy a deep breath is all what is needed, if the rebreather has a lung-controlled addition valve. So an SCR-hover is lung mediated buoyancy control: breathing with tiny breaths in the area of maximum exhalation and having the counterlung(s) as empty as possible (without triggering the addition-valve) means minimum buoyancy, breathing with tiny breaths in the area of maximum inhalation and having the counterlung(s) as full as possible (without triggering the overpressure-valve) means maximum buoyancy. Don't wear too much lead, it would clog your efforts to hover. You should start your training with the same total amount of weight as you are using for OC-diving with a 10 liter tank.
Although on Oxygen Rebreathers without constant flow as well as on passive-addition SCR's like the Halcyon there is no need to get rid of nozzle caused over-volume but the need of lung-triggered addition the way to hover is quite identical.
http://Rebreather.de/rebreather/scr-hover.htm © Karl Kramer, 19.June 1999