Here is what I think. I hope you like it.
"The PC300 can be turned off under software control..."
Means you can use a software switch in addition to the hardware switch. The hardware switch is the On Off button you push with your finger, usually on the front of a computer. A light switch on the wall is a hardware switch that does the same thing for a lightbulb, turns it on and off. A software switch does the same thing--turns the PC300 off--but instead of a button that must be pushed manually, it is a click of the mouse.
"from the Wireless Manager application or the signal strength icon in the status tray"
This is where you find the software switch.
Student or Learner