The cap can be used for testing the turbo hoses used on Audi and perhaps other VAG cars. The caps fit on the hose that connects to the throttle body.
To mount the cap, remove the clip from the throttle body, pull out the hose, put on the cap and put the clip on to secure it. Make sure to limit the compressor pressure to a maximum of 1 Bar (it may be hazardous if high pressure is used!). Make sure you know what you are doing before trying this.
You probably need another cap for the other end of the hose/system or try closing that by hand.
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