either way it goes directly into the engine. If you pull a vaccum line off it will not run in order to have the engine burn off the access cleaner, opposed to when you use the intake boot way the engine should still run fine and you can clean the manifold that way! My next question is what do you want to clean? the intake ports and manifold or the combustion chambers? Search this in the header above and read this postm, it might help How to clean your whole intake system and your injectors too.