I'm not familiar with the term "sympathetic" harmonics, but here's a great definition of "pinch" harmonics:

http://en.wikipedia.org/wiki/Pinch_harmonic

It's basically the "squeal" sound you get when you use the fleshy part of your thumb in conjunction to picking a note really hard.

The other kind (just holding your finger lightly over the 12th fret & picking, for example) are usually called "Natural" harmonics. Maybe that's the same as sympathetic?