Several new usb devices have their proprietary windows drivers onboard, especially wan dongles. when plugged in for the first time, they act like a flash storage and start installing the driver from there. if the driver is already installed, the storage device vanishes and a new device, such as an usb modem, shows up. this is called the "zerocd" feature.
on debian, this is not needed, since the driver is included as a linux kernel module, such as "usbserial". however, the device still shows up as "usb-storage" by default. usb-modeswitch solves that issue by sending the command which actually performs the switching of the device from "usb-storage" to "usbserial".
this package contains the binaries and the brother scripts.