The Quickening in the Highlander series and movies is characterize as magical lightning which transfers the knowledge, skill and "power" of a defeated Immortal to the victor.
Generally speaking, this is someone nebulous; while many Immortals have been slain by Duncan MacLeod, we've rarely seen him use the abilities or skills of others, and his strength and speed seems to be the of a normal, but highly athletic and very fit man in his early 30s. So there are very few indications that Immortals get cumulative strength, speed, stamina, physical ability or mental power stat increases, though each battle seems to make them more canny and slightly sharper in dealing with future threats.
In the series, we have evidence of it occasionally having specific magical affects, such as when he vanquished the immortal who killed his Native American wife and child [he was shown taking in the Quickening, with no pain, in a meditative position, floating slightly off the rooftop while an image of this family/their souls in the afterlife were shown]. However, generally, Immortals do not seem to gain any notable super-human abilities with each kill.
That being said.... has there been any evidence, either in movies, the show or any other media, how Quickening may affect normal mortals who may get caught in it?
Would they be granted some of the Immortals' knowledge or power, or would they simply be electrocuted?