http://wh40k.lexicanum.com/wiki/Shokk_Attack_Gun
The gun itself is one of the most bizarre weapons produced by the Orks. It projects a forcefield through the warp, beginning at the end of the barrel and ending wherever the gun is aimed (roughly). The portals look like small spinning black holes. In this way it is possible for living creatures to travel through the warp towards the exit point, much like a temporary Webway portal. Being of Ork construction, the gun is known to misfire causing the tunnel to implode, killing anything in transit at the time. Also, the journey is extremely frightening and both Orks and Gretchin have learnt to avoid being fired.
Snotlings are used as they do not fear anything until they are face-to-face with it, by which time it is far to late to turn back. This has the effect of terrifying the Snotlings, making them exit the portal in a frenzy, ripping and tearing at anything they encounter as they leave the portal. This is effective because the Mek often tries to aim the gun directly on top of enemy soldiery, and they emerge inside living creatures or enemy tanks(ripping them apart from the inside)as well as landing nearby due to the very effective condensed exit point. Otherwise they are seen dropping out of the sky onto unsuspecting enemy troops. The gun is maintained by a Mekboy and a Runtherd entices Snotlings into the barrel.