Sabotage is a temporary global Stat, which affects the chances of enemy weapons being jammed at the start of the wave. It's the only global stat which is improved by a sole skill owners, namely Saboteurs, which boost it by +5 initially, and by additional +1 with each new level. Once saboteurs are present and not necessarily deployed, the stat is boosted, vanishing after all saboteurs are dead.
The chance of success is built based on saboteurs' levels. With a number of enemies equalling your specialists' levels, they are ~50% likely to succeed, more with less hostiles, and vice versa. A detailed breakdown of values and chances under way can be found at the Talk Page.
Successful sabotage is shown with a 'sab !' tag. It is possible to sabotage all of weapons carried, not only ones wielded. For some reason, Vehicle armaments are noted to never be sabotaged.