Thanks for your answer.
The coupling of the oscilloscope is in DC, and the wave output should always be positive(offset DC), and a range of 9V or more, however, is 3,6V peak to peak and AC.
The output wave of zaper without connecting to the wristbands is the following:
When connected to the wristbands square wave disappears and turns into a train of pulses of 1V amplitude (as if the zaper neither could deliver current output).
The zaper is totally new and do not know if this is their normal operation.