You fit them on the piping between the actuator and something else.
If you are not careful you can have someones eye out with one.
If your car blows up it is set wrong.
GTiRich's advice is sound. The electronic ones are far better and more reliable and less likely to result in a dead engine. A Bleed valve is about 20-50 quid an electronic one is 300-500 but you get what you pay for.
Boost controller as stated above is much better but if a bleed valve is all you can afford then its a cheap way of getting more speed. Just make sure its set up right. I had one on mine for over a year and it was fine
I just fitted my bleed valve and I'm well happy, only a mild 10 psi. I waiting til I have the readies for an EBC. It also got rid of a stutter the car had while on boost (not all the time) because we bipassed the factory soleniod. We think it was shagged. :lol:
If you do go for the cheaper option i would go for the Dawes Device instead of a bleed valve. Spool up for the turbo is better and also if you order it striaght from Dawes themselves there about 15 quid delivered from America. I've had mine on for a year now and had no problems set to 1.2 bar. You'll notice a alot of difference. Oh if you've got a standard clutch say goodbye to it!!!