The idea of the freewing interface is to provide a way to connect the PC to your Action Replay (AR) or Gameshark (GS) cart thru a parallel port when there is no extra ISA slot on your PC to fit the comms link board.
The chain would look like this:
PC parallel port <-----> Freewing interface <-----> GS/AR <->PSX
5 vdc power supply -----------^
If you have an Xplorer cart you do not need this.