How do these USB KVMs work?

I need a USB KVM switch, and am about to buy an Iogear GCS42UW6 (unless someone suggests it's not good), but I'm not sure why it works for the USB parts. The mouse & keyboard USBs plug into the KVM, fine so far, but there's only one USB per computer going from the KVM to each computer. So how can one USB work for a mouse and keyboard? Is the KVM acting as a USB hub?

To see a photo of the entire package goolge: Iogear GCS42UW6 Compact KVM Switch tigerdirect

Thanks,
Paul

Hi

we use these in Lidl stores and they work fine, the KVM switch merely moves the mouse and keyboard between 2 outputs, windows works out that they are USB devices and installs them them accordingly.