From technology acquired from EXOS, Inc, Microsoft then released a force feedback product called the Force Feedback Pro. Built on the design of the Precision Pro, the Force Feedback Pro differed only in the inclusion of motors for the force feedback effects, and the lack of USB compatibility. (A DIY converter project exists.) Due to the inclusion of the motors, the Force Feedback Pro was significantly larger and heavier than the Precision Pro, making it easy to differentiate between the two.
As the PC joystick port is input-only, the only way for data to be sent to the joystick (to trigger force feedback events) is to use the MIDI capabilities of the port. This extension to the original gameport, first popularised by Creative Labs in their early sound cards, was intended to allow MIDI instruments to be connected to the joystick port but is used here to provide bidirectional communication with the joystick instead. Force feedback events are triggered by messages on MIDI channel 6, with effect data uploaded via SysEx messages. This means that force feedback would be unavailable on the earliest of PCs, where the gameports lack MIDI functionality.
Source: Wikipedia, "Microsoft SideWinder", available under the CC-BY-SA License.