#1
What's the most efficient way to play a harmonic? (the screeching noise, very cool sound) I've heard two different ways: touch the string above the metal thing and pick or picking and touching your thumb on the string immediately on the downstroke.
#2
Sounds like you're describing two different kinds of harmonics here.

A natural harmonic is basically the type you make by picking a string whilst touching the string over certain areas like the first one you mentioned.

An articial one is like the 2nd thing you mentioned, which is the squealing one. It's a bit more complicated than that and requires some practice.
Essentially you pick the string then touch it with a tiny bit of your thumb in the same movement.
Of course it isn't always going to work that simply. There's alot of little factors that contribute to it working time and time again.
#3
They both work. Though the natural harmonics (just playing above the fret) is easier to do.
#4
my harmonics are pretty inconsistent. I use my thumb. I've seen people use the joint in their bent middle finger to create a artificial harmonics too. is this common?
#5
Thx for the replies... are there certain keys that you can't use harmonics with? And is the second harmonic mentioned the most commonly used in rock songs?