ive seen some amazing guitarists like andy mckee and tommy emmanuel do them...but how???? tips, links, or any help at all is much appreciated. thanks
They are known as slap harmonics. They are actually alot easier than they seem, and, while a little difficult to be consistent with at first when actually playing songs, with only a little practice they are a great way to either get in harmonics with a more percussive attack or a way to produce harmonics quickly without the time taken to adjust the left hand.

Start with a harmonic on the 12th fret. Don't pluck any strings, and with your right hand, sort of snap/flick your hand at the strings so that the tip of your fingers (only the tip touches the string) hit RIGHT ON the 12th fret (not the space between the fret where you normally put your finger to play it, but on the metal fret itself.) Once you hit it, let your hand bounce off quickly. If done right it should produce a harmonic. It takes a little practice to be able to do it every single time because you have to get your hand off the string fast enough and you have to be accurate enough about hitting them right on the fret of the harmonic you want to hit. It can be done with other harmonics than the 12th fret harmonic, but that one sounds so easily that it is the best to start with.