Pishing is a birding tool that I have not quite mastered yet. The basic pish is a sound like pshhh, meant to resemble the scolding calls of chickadees, titmice, and other small songbirds. When birds hear this, they will frequently come to investigate the source of the sound and determine if there is a real danger involved. Dunne also suggests a screech owl call and dying starling noises as alternate sounds to catch birds' attention. My best success has been with a chip note that sounds somewhat like a common yellowthroat.
A good pish will come close to sounding like a real bird. Bad or overagressive pishing may end up driving birds further into the brush.