Its makes connecting vacuum gauges to your bike very easy. 

On quite a lot of bikes connecting adapters is very awkward (on some YZF750s you have to remove the carburettors to connect the adapters). 

With the extension kit you put the adapters permanently into your bikes inlets and connect the rubber hoses to them. Connectors are pushed into the other ends of the hoses to accept the rubber end caps. The ends of the hoses come out at any convenient place you want. 

When you want to check your carburetors all you have to do is remove the accessible rubber end caps from the hose connectors and push your gauge hoses onto the connectors. Start your engine and take a reading. Very simple, very quick.

All you do is connect the vacuum gauge hoses to the fitting on the needles and then push the needles through the inlet rubbers to take readings. Self seals when it comes out. As used by Kawasaki and detailed in genuine Honda service manuals.