The MAC valve is good for holding up to 120psi,the GM solenoid is good for around 15psi,anything close to it or above that,it starts to leak like crazy.
The MAC valve is built like a tank and the coil will never burn out,it's made for industrial use,the GM part is,well made by the lowest bidder with the cheapest material.
The MAC valve,if it fails,it fails shut(you will be running on wastegate mechanical pressure on whatever it's set on).The GM solenoid,when it fails,it will fail open(meaning your turbo will overboost & hit maximum psi ).
Motec,AEM,Hondata,ECUtune,Greddy,Apexi,Turbosmart, Chrome and many other companies that build electronic boost controllers and standalone ECU's all use to use the GM solenoid,but now they all use/sell a rebranded MAC valve.
In a nutshell, it intercepts the pressure signal going to your wastegate/actuator. By not allowing any pressure to reach the diaphragm in the wastegate/actuator, the wastegate stays shut. Once the expected boost pressure is being reached, software triggers the solenoid to open up so the wastegate/actuator sees the pressure and opens up, which allows exhaust energy to bypass the turbine and slow or stop the compressor from increasing the boost level.
After watching above video: You connect the solenoid like you would a MBC, between a boost source and the wastegate actuator.
If the valve is closed, no boost can make it's way to the wastegate actuator and you will make unlimited amounts of boost, as the wastegate will not open.
Since the EBC is controlled by computer, you can set the RPMs you want to have the valve do stuff. This is why we can see better spool with one, we can keep the valve closed to the very last second. This inturn keeps the wastegate shut and the turbo to spool faster. A MBC or straight boost line to teh wastegate actuator will cause the wastegate to crack open slightly as the boost increases.
You control the duty cycle of the valve, so it opens and closes very rapidly, thus lowering the pressure reaching the wastegate actuator. This allows you to pretty much achieve whatever boost level you'd like over the stock wategate actuator preload.
Think about it this way...you have a 5psi wastegate actuator. Once your turbo makes 5psi, the boost pressure is greater than the preload on the wastegate and it intrun moves the rod to open the wastegate. The diversion of the exhaust gases through the wastegate and not the turbine causes you to limit the boost to 5psi. When you use a EBC and want 10psi. The turbo pumps 10psi, but you controll the rate of boost entering the wastegate actuator so it only "sees" 5psi. So while you acutally are making 10psi now, you're controlling the wastegate in such a manner to maintain 5psi over the stock pressure.