Bussing is the act of distancing yourself from your scum partners, usually by helping to lynch them (and hopefully playing a major role in convincing others that they are worthy to be lynched). The term "bussing" comes from the phrase "to throw under the bus", which means sacrificing them to gain credibility. The 'bussee' is the player being thrown under the bus, and the 'busser' is the one doing it.
The effectiveness of this technique varies, mostly because it has been around practically since the beginning of the game of Mafia.
Why bussing works
Bussing works because (if done correctly) it distances the players doing it to each other sufficiently that nobody (or at least, nobody but the smartest players) will ever suspect that they are both scum in the same faction. The reasoning applies that if the bussee is actually lynched; the busser is considered to be obviously pro-town. Because, of course, why would a Mafia member try to get his own scum partner lynched? And thus, WIFOM ensues.
How to spot bussing
Suspicion commonly arises when someone is suddenly lynched for what seem like minor or spurious reasons, and yet flips scum, because it may be indicative that one of the major proponents of their lynch is actually one of their scum partners.
If a scum player is about to be lynched (or feels like it might be a possibility that they will be lynched), another tactic is to bus their partner on their way down, trying to get them lynched as well. Often, this includes quite a bit of bussing on the part of both parties, but when the original scum player is lynched, his bussee is often (ill-advisedly) considered to be a guaranteed townie. The argument being: why else would the now-dead Mafioso have tried so hard to get them lynched?
Due to the prevalence of reverse bussing, care must be taken in evaluating the reads and pushes of lynched scum, as their strongest suspect will occasionally be their partner.
When two (or more) players are the leading Bandwagons for the day, and they both happen to be scum of the same faction, they may (and often do) engage in double bussing. Both of the players begin to attack the other in any way possible, attempting to make their attacks appear to be an effort of self-preservation. When done skillfully, double bussing can sometimes cause the town to mistakenly assume that it is a town-on-town fight, and shouldn't be taken seriously.