The M-18 Taser is the Civilian version of the LE M26. I'm a Taser instructor and here's the scoop:
Voltage Smoltage. The "18" and "26" are for the number of watts that the device is putting out. The human nuero-muscluar system is run on about 15 watts and most "stun guns" run at under 10, regardless of voltage. This means that if the person was tough enough, they could basically fight through the pain of the stun and still control their muscles. With 18 (or better yet, 26) watts ripping through, the electrical impulses from the brain are disrupted and over-ridden. All that sounded great during the course, but I actually got shot with one of them and realized what that all meant: You can't do anything but submit. Without voluntary control of whatever muscle group is being "zapped," the target is helpless. Did I mention the pain? Oh yeah, the Tasers still have the pain compliance aspect of traditional stun guns as well... but more so. I've be "zapped" with the old versions too and they hurt, but there isn't much comparison. As in the example that MP5sd noted above, if you are tough enough, mad enough or stoned enough you could get through it because your muscles would still be getting signals from the brain.
The standard shot results in 5 seconds of 10-15 pulses-per-second. If the trigger is pulled again, the weapon cycles for another 5 seconds. Fully charged batteries (or relatively new non-rechargables) should give 10 consecutive blasts. The weapon can be used to "drive stun" if the front end is pushed against a target after the cartridge has been fired (yes, both the target with the probes and the target being touched feel the effects, though reduced slightly) or without a cartridge attached.
Of course, no weapon is magical. You still have to hit the target with the probes (although a circuit can be completed with only one probe in the target, if the other wire lead touches him or he is on a particularly conductive surface. And the energy will only arc over a little more than 2 total inches (ie- one probe in, the other stuck in a leather coat still should work... both in the coat with a thick sweater underneath is iffy though...). The M-18 probe cartridges have a range of 15'. They use compressed nitrogen to launch the probes and are not considered firearms like some older models (except maybe where airguns are considered firearms (NJ, etc...). The wider the spread on the probes, the better (the more area disrupted), so drive stuns are not as effective as shots from 10'. There is an 18 degree offset on the trajectory of the probes, to create a spread between them.
Major muscle groups are the intended target areas: back/shoulders, thighs, chest. In some areas, LE is required to have the probes removed by EMTs, but that is a CYA thing. They are barbed pins that can be yanked out without causing any significant damage. Mine were.
While I am rated as an instructor, I do not sell these things, nor have I ever gotten paid to certify anyone... ie- no agenda, I'm just a believer and a proponent, particular in LE use as an alternative to pepper spray and hands-on techniques under many circumstances.