What is the difference between open and closed dual head "air chucks" (the tools you plug into air compressor hoses to inflate truck tires)? I insist I want to get one from one of the big name tool companies like Snap-On, Mac, Matco, SK, ect. Because the ones in the autopart stores really are bad. It's confusing because the different "air chucks" designated as open and closed look the same???