If you search the topics you'll pick up a few threads about this. Basically its a small device that can be fitted even by the mechanically inept (that's me!) into the wastegate vacuum hose and allows you to bleed away (turbo) air pressure and thus fool the system into giving you more boost than standard. It delays the opening of the wastegate. Its is a very easy way to add about 10 - 15bhp, although if you set it way too much it might lead to damage.
That's the very non-technical answer. Others will be able to give you the full detail.
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