The Amal valve is a crude method of boost control. It operates by diverting boost pressure away from the wastegate actuator and hence can safely increase the turboboost. The valve is fail safe, hence it is powered to divert boost away from the actuator and to the airbox. If power is removed or the wire is broken,the Amal valve will close and all the boost pressure will be passed to the actuator.
There are three pipes which attach to the Amal valve, each pipe is identified by a letter on the Amal valve, C(middle) comes from the turbo housing, W(top) goes to the actuator, R(bottom) goes to the airbox. The purpose of the Amal valve is to increase the build up of boost but not allow the overboost control to operate. Hence as boost builds up to a pre-determined level within the boost control ECU the Amal valve will be powered, diverting pressure away for the actuator. When the boost reaches a certain level for engine speed and throttle position (load demand) the boost ECU will remove the power to the Amal valve and the wastegate actuator will be used to control the boost pressure.
The operation of the Amal valve will usually result in the boost overshooting the pre-set value of the actuator as the boost initially comes in. The overall steady state condition boost pressure is not affected by the Amal valve for constant engine speed and load. A simple two stage boost control, high and low, can be implemented by using a switch to the Amal valve and stronger actuator. If the Amal valve is powered the turbo will operate in high boost mode, if power is removed the turbo will operate in low boost mode. The Amal valve can be configured for variable boost using a simple mark to space ratio device which can pulse the valve on and off for varying lengths of time.

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Understanding what the Amal valve is and what it does will probably help.
Firstly a standard turbo wastegate actuator will open the wastegate at about 5 PSI. The Amal valve is a 3 way valve with a solonoid that opens up a larger passage to one of the outlets.
In normal operation the valve is actuated allowing a certian amount of control boost to bleed away from the actuator which in turn makes the actuator slower to respond to boost and later to ‘actuate’ or open, allowing in normal operation about 9PSI of boost to be made.
If the ECU senses a problem with the engine it will de energise the AMAL valve and lower the boost back to the actuators basic 5PSI.
The AMAL valve has no effect on engine running apart from the amount of boost, so if you remove it you need to replace it with another type of bleed or controller OR a stronger actuator, the only pitfall is the ECU will no longer be able to knock off a few PSI if the ECU senses a problem.
On Chipped Cars the AMAL valve has limited limp home effect and has been known to blow apart, stick etc so they are often dumped, most tuners just rip the guts out of them so the car still looks standard.