Diesel tuning is very dangerous to amateurs. As more fuel almost always results in more power, some people can get carried away with it.
Unless you have the proper equipment (boost gauge and EGT minimum) I would strongly urge you to either leave your chip on a minimal gain setting, or have it setup by someone with the proper test equipment.
Fuel Rail Pressure Relief Shimming and Removal:
One end of the common rail houses a pressure sensor, the other end always houses a pressure relief valve. This is simply a ball and spring setup on most vehicles.
It is possible to place a small shim underneath this spring which will effectively increase the amount of tension the valve is placed under.
The higher the tension, the more fuel pressure that is required to lift the valve off its seat.
It is there in the event of an engine control mishap causing excessive pressure in your common rail.
Each manufacturer destroys many systems to find the absolute pressure limits.
Some manufacturers relieve the fuel pressure well before this max limit, other relieve much closer to it.
There is a couple of vehicles that can be shimmed and kept under their maximum working pressure,
And, there is lots of vehicles that cannot be shimmed even slightly without lifting the pressure past the limit.
As has previously been discussed modification of this valve is not recommended for the majority of people.