A Bleed Valve does what the name suggests as it bleeds off boost pressure before the actuator, it fools the actuator into believing that its running lower boost than it really is. The problem with bleed valves is that although cheap they raise the boost pressure without ensuring that the ECU is making correct adjustments to the fuelling, which can cause the engine to detonate leading to premature wear and engine damage. They also remove the boost limit, rendering the ECU unable to shutdown the engine if a problem should arise.

Its a pikey cheap way to increase boost and although many people will tell you there is nothing wrong with them reputable tuners in general use a different approach to raising boost. I would not use one