Okay, while this is not a mod I'm going to do on my truck, I've heard it asked and here is how you do it.
WARNING: You need to have gauges BEFORE you do this, your EGT's will come up fast under low boost conditions.All you have to do is splice into your MAP sensor.
Basically the map sensor senses how much manifold pressure is in the engine and puts fuel in accordingly. The sensor line is fed by a 5 volt line and the more pressure in the manifold the more voltage is pushed down the signal line, so when you install the switch, you are pushing maximum voltage or a full pressure signal to the computer, dumping extra fuel in until the turbo gets up to speed.
For the 4-pin connector, you only need two wires. If you look at the map sensor plugged in, facing the cab, you want the two wires on the right. The colors vary from year to year, but the two on the right (as you face the cab) are what you need.
Put a line splice on both lines and just wire up to a toggle switch that has on/off capabilities and you are done. Yes, the MIL will always stay on because the sensor is getting full voltage and the truck doesnt like that.
Once the switch is turned off, it takes 3 - 4 cycles for the MIL light to go off, unless you have a programmer to clear it, but you will cause the light to come on.
You better have gauges because if used improperly, your EGT's will skyrocket.
Also, be careful where you use this. Doing it on the street and pissing people off only makes it bad for all of us.