In a nutshell, the PacDrive fires 5v to the hopper relay, which gets the hopper spinning. However, it stays on until the opto senses the coin output, and in turn fires a relay, which connects 2 pins on your Ipac, which makes the "[" or "]" key press on your PC.
For triac payouts, you can do away with the Ipac as it does exactly as you say. I use an aduino R3 and route the opto back in to the arduino to stop the hopper spinning.