The wastegate is mechanically set to effectively dump exhaust gases around the turbo’s exhaust impeller. This action effectively stabilizes the boost at an intake manifold pressure of 10 PSI.

The BCS is an alternative pathway for air pressure, AWAY from the wastegate. This means it is a "wastegate signal bypass" system. The air pressure port, on the side of the turbo’s compressor outlet is configured as a "T". One port of the "T", containing the boost pressure signal from the turbo’s compressor outlet is fed to the wastegate actuator diaphragm. The other port on the "T" goes to the BCS.

When the BCS is actuated, electrically, by pin 32 of the EEC, it opens, allowing the boost pressure signal to be diverted through the other port on the BCS, to the turbo’s compressor housing inlet. The turbo inlet is a low pressure area, not under boost pressure. This effectively "bleeds" off the boost signal, that the wastegate actuator has acting upon it’s diaphragm.

So, with the BCS energized, the wastegate only gets 10 PSI on the diaphragm, even though there is 15 PSI of pressure in the intake manifold.

The manual method of raising the boost is to replace the BCS, with a device with a bigger opening than the stock BCS. This bypasses more air away from the wastegate, allowing the engine to develop more boost.

You can use an adjustable regulator valve, or items with a fixed internal diameter. Adjustment is made until the desired amount of boost is reached.

Actual methods to raise the boost, pick one.

1. I like the "Blue Butt Splice" method of raising the boost. It's cheap and works. Replace the BCS with a standard blue "butt splice" connector from the crimp-on electrical connector section of your local parts store. If you don’t get 15 psi, use a yellow one instead. This works well on automatic cars, because you wind up with slightly lower boost pressure, than the following methods.
2. On later cars, you can just loop the hose that goes to the BCS, back to the turbo inlet nipple. This effectively eliminates the BCS. Ford in their grand wisdom, made the port hole in the "T", smaller, on the later cars, and it winds up having about 17-19 psi of boost. Most cars should be able to run this way 9 months out of the year, even without an intercooler.
3. If your car's model, never had a BCS, then you can duplicate the stock configuration, and substitute a valve or regulator for the missing BCS. If the nipple on the compressor housing outlet is not a "T", then you can just use a hose barb "T" in the wastegate hose. Always route the "bypass air pressure" back in to the air intake system, after the VAF meter. This air was already measured by the VAF, and should be accounted for, not let to escape in to the atmosphere. This could adversely affect the fuel mixture.
4. "The Digital Boost Controller" Are you envious of your friends, with their HKS, or Greddy digital boost controllers? Then make this one, out of parts that are already on your car! Find the EGRV solenoid. It is an electromechanical solenoid that switches the vauum on and off that supplies the EGR valve. On Fox chassis cars, SVO, TC etc, it's on the passenger side fenderwell, between the strut tower and the firewall. It has a 2 hose vac connection and a 2 wire electrical connection. It is mounted on a bracket with another device, the "overboost warning buzzer" switch, that looks like a big bullet with RV on the rounded end, pointing up. Study the wiring diagrams for your car, and repin a few connectors inside the car. The O'boost switch heads for the multifunction buzzer near the glove box. The EGRV is wired to pin ?? of your EEC. What you are basically trying to do is shown in the following diagram.

You can usually find a spot in the wiring at the kick panel, where you can just move a pin to have the connection made by the stock wiring. This is preferable to cutting and splicing thge stock wiring. Spend some time figuring it out. It's worth it, and also, very stealthy. The only real giveaway is that you must move the EGRV solenoid closer to the turbo. I keep both pieces on the bracket and move it to about where the stock BCS was. Keeping the hoses short, eliminates boost overshoot in the lower gears, and when you first get on it.

You can pick the "goo" out of the top of the overboost switch and then adjust it. They're usually set to 17.5 psi. 

With this mod the wastegate never opens until the boost setting is reached, then it slams it open. Drivability is GREATLY improved with this mod, since you get more boost lower in the RPM range.