The electricity generated from piezoelectric materials is momentary charge (or voltage) when the material is initially stressed, or released from stress.
The voltage "squeezed out" of piezo-electric materials is in proportion to the mechanical deformation (stress) placed on the material and the intrinistic characteristics of the material. If the force producing the mechanical deformation on the material is reversed, the polarity of the output voltage will also reverse.
Figure 1 shows how a piezo-electric (dipole) molecule generates voltage. The plus and minus signs illustrate the bipolar charges within the molecule.
In piezo-electric materials most of the bipolar molecules are oriented in the same direction. Voltages produced by piezo-materials is not constant, as in voltage produced from a battery. Instead the voltage raises and decays rapidly like a pulse.
The time constant of the voltage pulse is determined by the piezo-electric material's dielectric properties and internal resistance, see Figure 2. Voltage decay is also influenced by the resistive-capacitive (RC) properties of any external device (ie: oscilloscope, etc) connected to the piezoelectric material.