3. Infrared system: instead of using a PC cable connected to the camera can use an infrared photocell.
This shoe is placed in the chamber that is at the top of the chamber.
When the flash fires infrared photocell triggers that will make the other flashes firing. In the other flash has to be a fitting recipient of this light and have the flash shot.
4. Radio system: They are a system and need to put the transmitter on the camera and the receiver flashes.
This way when you shoot the camera flashes going to shoot. Is the only system where it is not necessary that the receiver see flash since it works with radio waves.