Float is a bad term really. The float position means that the ground is disconnected, like a cheater plug would do. Semi float could mean what ever the manufacturer wants it to mean. However it usually refers to a situation where circuitry the the primary side of the mains transformer remains grounded, but the ground to anything supplied to the secondary side is lifted. He should look at his user manual for details, and locate one if he does not have it.
Unfortunately grounding power amps, often leads to ground loops. As long as he experiments with this switch when the amp is connected to other grounded equipment by grounded interconnects, there would be no risk of shock. Unless one knows what his system hook up is it is hard to comment. The most common source of ground loops are cable TV and internet connections.
The cause of ground loop hum and buzz is always due to a resistance between grounds, without exception. So getting all equipment to the same ground potential is always the cure.
