Guitar players have many techniques in their arsenal. A guitar is a very versatile instrument and has the potential to make many different sounds, few more distinct than the pinch harmonic, also known as pick harmonic, or a squealy.
The basic technique behind it is that right after you hit the string, the string hits your finger. This cancels out the fundamental frequency and all overtones, except those that have a node at that location. Overtones that are a multiple of the intended overtone share the nodes of the lower tones. The Physics behind sound waves is extremely evident in all musical instruments, and pinch harmonics are a great way of manipulating physics in order to create a cool distinct sound that has changed guitar playing.