A quick search for "boost creep" will give you all kinds of answers.

In the event you "can't find it", here's the basic rundown:

An exhaust restrictor is exactly that. A restrictor plate you put in the exhaust. Pick an exhaust flange and buy 2 gaskets for it. Buy some sheet metal, probably 16ga steel would be fine. Trace the outside of the exhaust gasket on the metal. Now trace the bolt holes. Cut the piece out, drill out the bolt holes. Now you have a couple of options. Start with a 2" hole in the center, or just drill a whole bunch of smaller holes like swiss cheese. Install the plate at the flange, using an exhaust gasket on either side. Ideally, you'd have the hole (or holes) large enough to provide good flow, but still enough restriction to prevent boost creep.

Imagine the turbine (hot side) spinning inside a housing. The exhaust is coming out at a certain speed. The exhaust downstream of the turbo (stock) has a certain amount restriction. When the boost pressure reaches a certain amount, the wastegate opens. The wastegate is a passage that opens up to allow exhaust gasses to bypass the turbocharger by dumping exhaust around the turbo. Without the restriction of the stock exhaust, the exhaust gasses find it easier to go through the turbine instead of going through the wastegate when it opens, thus spinning the turbo more, thus compressing more air, which the stock ECU can't keep up with. Sort of like asking would you rather drink through a coffee stirrer, or a McDonald's straw. So if the wastegate passage were bigger, it'd be more effective at preventing this "boost creep". Knowing what you know now, the definition of boost creep is when the wastegate opens, and the turbo continues to increase the boost pressure.

This is such a great writeup:
FC3S Pro v2.0: How-To - Zenki wastegate porting

I think it's time to read. A lot.

 

double post... weird