parazapper
You are partially correct but that claim misses some vey important points.
#1 Electricity takes the shortest path which in this case would be from one terminal of the battery, across the tongue, to the other terminal, thereby missing over 99 percent of the body.
#2 The zapper works not only on the voltage applied but also the pulses applied so that the battery would necessarily have to be repeatedly touched and then removed from the tongue.
#3 this can not be done just once, it must be repeated for a minimum of 7 minutes, then rest for about 20 minutes, and then the process repeated at least 2 more times.
#4 more than 2 contacts improves results and 4 contacts appears to be a good choice with only 1 negative and at least 3 positive at the extremities.
#5 the larger the contact area, the better the general results that can be expected.
There are many other considerations such as multi-frequency will definitely produce better results.
Using the 9 volt battery in this way in any emergency situation is definitely better that doing nothing, what do you really want, cheap or the best possible results?
http://paradevices.com/how_to_zap.html see battery tapping