Basically, you intercept the pressure going to the bottom of the BOV so that at high boost, the BOV only sees pressure on the top, forcing it to stay closed. But when you lift off the throttle, pressure is again applied to the bottom to help open the BOV more quickly.
I used the nitrous controls in DSMLink to control the fuel pressure solenoid. Basically, you feed a pre-throttle plate pressure source (“A” port on a 2G TB or the “A1” port on a 1G) into the “input” side of the fuel pressure solenoid. Then feed the switched output to the bottom feed of the 1G BOV. Using the standard nitrous controls in DSMLink you can then have the BOV completely shut with high throttle, but completely stock at all other times. The picture below illustrates which port on the fuel pressure solenoid goes where.
If you're going to run more than, say, 20-21 psi with this mod, and you plan to do more than drag race, then you'll need to pay a little more attention to the throttle activation point. In my case, I wanted to run the car at the road course with spikes to 23psi, holding a pretty constant 21psi after that. The problem is that the 1G BOV will open on its own around 19psi. I couldn't just plug in a really high TPS value into the nitrous controls because there were plenty of times I was modulating the throttle below that point but still building more than 19psi. Just using a high TPS value resulted in the BOV opening at odd times, defeating the purpose of the mod.
To work around that, simply find the TPS point needed to build, say, 18-19psi on your car and set the activation point around that. For my setup, I was hitting 20psi with 45% throttle above 3800 RPM in 3rd gear. So I set nitrous controls to 40% TPS and all worked well. Now, above 40% TPS, I have a BOV that will stay closed no matter how much boost I build but will open like stock below that.
NOTE: I want to point out that this still isn't an ideal control mechanism. Ideally, you'd want to have a differential pressure sensor wired across the throttle body to control the BOV properly. But triggering off TPS at or around the cracking point of the BOV seems to suffice for the most part.